Boots of Speed Are One Example Where It's The Same Price
They didn't divide those up into separate effects in the guidelines for creating magic items. So the only way to determine this is to look at some examples and try to see what they're doing.
Boots of Speed let you get Haste for 10 rounds a day, split up any way you wish, for 12,000g. They got that price as follows:
Haste (10 rounds), spell level 3, caster level 10. That gives us a cost of:
3*10*2000 = 60,000. But that gives you five charges of a CL 10 casting, and the boots only work for 10 rounds (a single casting). Thus, to get one charge, divide by 5. Final price: 12,000g.
That's an exact match. I'm sure you can find cases that don't match perfectly, but this shows that they don't place an extra gold value on splitting the rounds up.
Pricing Should Be Determined Individually For An Item
Why do they do that for some items and not others? Item pricing is an art and the rules flat out say that in the rules:
Not all items adhere to these formulas directly. The reasons for this
are several. First and foremost, these few formulas aren’t enough to
truly gauge the exact differences between items. The price of a magic
item may be modified based on its actual worth. The formulas only
provide a starting point. The pricing of scrolls assumes that,
whenever possible, a wizard or cleric created it. Potions and wands
follow the formulas exactly. Staffs follow the formulas closely, and
other items require at least some judgment calls.
When determining the price of an item, you can't just use the formula given and call it a day. The prices in the DMG are hand-edited after using the formula, as the formula is itself a guideline. If you think being able to split the rounds up into multiple uses suits the item, do it. If you think it's too powerful for a given item, then don't do it, or raise the price appropriately.
In the case of Boots of Speed, would you pay 12,000g for a 1/day Haste1? With the rounds being able to be split up, it can be used over multiple fights and helps the item be more useful. IMO, it'd be rather weak if they didn't let you do that. It could be different for other spells (an item granting free action Righteous Might would be more powerful, for example).
1You can reduce that to 6,000g for a 1/day Haste that lasts 5 rounds instead of 10. Either way, you use it in a single combat and the item is now useless for the rest of the day. The item as listed in the DMG can be used in multiple fights if you don't burn all 10 rounds in one combat, which helps maintain its usefulness over the course of an adventuring day.