Yes, that's how that and all other flat-gold-cost enchantments work. Pretty simple.
That depends on whether or not your game obeys WBL.
If it doesn't, then yes, you should add transformative onto any weapon you're interested in keeping and probably also your butter knife (you did cast Masterwork Transformation on it, right?). 10K gp is a paltry sum for anyone with access to money-making magic, and basically nothing if you've got a skilled rogue. Transformative things make life easier in general for people so most of the time we get the general population's gear upgraded and standard of living soars. Warning: a proliferation of transformative implements makes murder mysteries a real pain in societies with privacy laws restricting the use of divination magics.
If it does but you're allowed to contract out weapon crafting and get custom items then it depends again on how WBL is enforced.
If WBL is enforced in both a positive and negative direction (so, when you use a potion, you find more gold to make up the difference) then selling things is never a bad idea because even if you sell them for half price your money has to add up at the end anyways so it doesn't really make any difference whether you sell the stuff or burn it and leave the remains in a ditch, you'll end up with the same gold total anyways. Assuming you can buy the items you want with your wealth, not having transformative is always better than having it if you only use it to turn an NPC weapon into your signature form, because you could always just transform the weapon by getting rid of the one you found and purchasing an item with the same enchantments with the only difference being the second one is 10K cheaper (again, assuming you don't make use of the other benefits of transformative). I have no experience with this ability in this subclass of game, but I see no reason it'd be different than enchantments in general in such games. Generally in this kind of game I sell everything and just buy whatever I want to end up with at the end, even if it means selling things and then buying the exact same thing because you're never worse off that way.
If WBL is only negatively enforced then transformative is definitely worthwhile on the first weapon with useful enchantments you encounter that is worth at least 20K gp (assuming you don't have magic weapon at this point you're already interested in keeping). This is because selling the item will lose you (permanently) half its value, whereas enchanting it loses you a flat 10K. Note that if you have an esoteric build it is unlikely you will encounter weapons that count as having 'useful enchantments' without collaborating with your GM. In any case, once you transformative something, you should decide whether or not to transformative future things based off of the loss difference between selling it v.s. transformative's 10K cost + half the value of your current weapon. It is thus unlikely switching weapons a second time will be worthwhile. The only time I did this the GM started putting transformative on the enemy gear afterwards, so it became moot.
As an aside, there are lots of good reasons to get transformative weapons other than being able to use the stuff the giant dropped. If your PCs, for example, have specialized in weapons that are "the same general size and handedness" it can let your PCs swap weapons/enchantment as tactically useful in different situations. This isn't really a big enough deal to affect whether or not it's worth doing in any given WBL but it was a lot of fun. If you're willing to all just have one weapon and toss it back and forth to each other between turns in combat, your tactics will become extremely convoluted but you can save a good deal of money on not having to buy weapons for every player. It's a bad decision optimization-wise, though, because if someone is prevented from making a hand-off the whole rest of the round is likely lost.