i have seen a seller on ebay selling them for about £40+ 15 for shipping to portugal but i don't know what motherboard it has, i also have seen people selling them for about £ 60 + 9 for shipping to portugal , but here in portugal there is a site kind of like ebay where the cheapest one is 75€ with free shipping but the screen is badly scratched, so i think that 50€ to 70€ is a good price
Certain bundles were sold and if the PSP is from one of those then you can know it's age, and there's also the date code under the battery... but if they don't know it's likely that don't care anyways and it wouldn't affect the price.
Go to ebay
Run a completed item search for a PSP 2000
Look at the numbers in green indicating they sold.
Get an average idea of the cost.
Depending upon how reasonable the seller is subtract a little as your transaction will be without all the associated ebay fees.
If the PSP is in good condition and is below or equal to Data Code 8B, it's worth around 60GBP/75USD, if the Data Code is higher, I suggest getting a PSP-3000 or Go instead - it's a far better deal than one of the newer PSP-2000's.