Any economist will tell you that a lot of factors can control the market price of an item. Here're a few that are easily applied to treasure:
1) Artistic merit
An elegantly shaped leaf blade, inlaid with fine etching in an ancient script, the scabbard carefully wrapped in red silk? Or a straight, boring but functional, enchanted longsword? The latter will sell for 'book' price... to a warrior who knows his swords. The former goes for a much greater - but much less predictable - price, to a collector.
2) Creator fame
Even a ten-minute doodle by Picasso is extremely valuable - if he signed it. A Masamune katana is not just a masterwork but a legend, with a value vastly exceeding the apparent value of the blade. Social cachet or reputation can change the value out of all proportion to the 'book' value.
3) Knowledge value
Books are obvious, but tend to be treated too casually in a D&D setting. In a pre-printing-press economy, a book represents months of years of labour, initially from a scholar and then from copyists. A book doesn't just represent knowledge, but exclusive control of that knowledge. (This is why the church kept the bible in latin for so long...) In a world with wizards, the value of a tome of magic depends greatly on the skills of the reader.
But books aren't the only paper treasure. Next time, instead of giving out 1000gp, how about a letter of credit for 1000gp from a major merchant house? In some ways it's more valuable (easy to carry, less obvious to thieves). But it's only as good as currency in areas where that house is known and respected. If you can't get to a factor of the house to redeem it, it can be traded to others who can - but what you'll get on the deal depends on how convenient it is for them to redeem it. And on how much that group's credit is trusted. (And maybe it's a forgery.)
That's an early-renaissance mindset. If you prefer a more medieval approach, maps, letters from the king and correspondence can all have high values to the right buyer. (For a real world example, in medieval France the bastard son of a noble isn't noble unless confirmed legitimate, such as with a letter from the sovereign accepting the evidence. That letter is priceless... to its legitimate owner, or to his enemies.)
4) Market size
Nothing is worth any money unless you have a buyer, and the size of the market restricts the value. In a remotely medieval world, the market is intensely restricted anyway... to nobles, and their closest and richest retainers. (Peasants don't have cash money; they can't afford it.) But some things have a more restricted market - which means the value depends on your interactions with the one or two people in an area who might actually want it. Otherwise it can only be sold to a merchant who can ship it long-distance to a buyer elsewhere.
Magical books are valuable, but only to wizards. (Or to people who want to see them destroyed.) An beautifully carved longbow with a 200lb draw weight is of enormous value... but only to the insanely rare man with the muscle to actually draw the thing. It's useless to anyone else. A detailed map of a key territory has great, but unpredictable, value to the landlord, to a tax collector, or to an invader - but nobody else. An ancient smith's notes are invaluable, but only to student smiths. A suit of armour is worth a lot less to someone who isn't the right size to fit it.
The favour of a king is a boon beyond price to his subjects. But it won't pay your rent - unless you ask for money, and therefore use up your favour. And how much money you get will depend greatly on the king's mood, and the state of his finances, at the time you ask. There's no reliable way to predict the results in advance, either.
In D&D, knowledge is power in a very real, fireball-throwing kind of way. Instead of a fireball scroll, give out a favour from a powerful but busy wizard. Or from a servant who can grant access to a powerful lord. (Bribing the staff in return for access was even more a feature of medieval politics than it is now...)
And in a fantasy world, favours owed by gods, demons or elementals are often payable on-demand in very convenient ways, assuming you have some means to contact them (which could be given to the party as part of the favour).
Fresh fruit is a staple of trade across time, but you have to sell it fast. Strange magical components may deteriorate just as quickly. The same applies to a lot of information. Political or commercial information is worth more the more recent it is.
An old and broken-down warhorse is worth a fraction of the price of a younger one with the same training. (And either of them is worth many times more than an equivalent horse without battle training.)
7) Local conditions
Swords are worth more in war zones. A good bow is worth more in a desert than in a welsh forest. Embargoed goods can fetch more... if you're willing to risk trouble with the law in return for the extra money. A valuable bolt of silk or decorative plate might fetch half its "normal" price, if a merchant ship with similar cargo has just arrived in town.
In standard D&D, magic is a fairly typical commodity. But unidentified magic still has a greatly unpredictable value. And in a low-magic world, not just the value but the uncertainty of price in a magical item can be a lot greater. (How much is a +1 longsword worth if it's the only one in the country?)
That can also change the value of otherwise mundane items, if a wizard needs them. (In Niven's Warlock's World, for example, raw gold stores magical power until a wizard uses it up; gold only has value to anyone else because possessing it indicates that a wizard owes you a favour.)