It would be better to buy the Playstation copy. Generally, you can find a copy of Valkyrie Profile on EBay for around $70-$100 for a bidding. It is significantly more expensive when doing a Buy Now purchase, upwards of $170. However, the PSP alone costs around $200($250 for the "Entertainment" pack) and the game costs $40.00. So, unless you are obsessed with playing games on a smaller screen just because you can bring it with you, it'd be better on the wallet to go for the EBay route.
Let's see. I'm not up to the minute with rare games but I own a few that have been named:
Suikoden 1, Suikoden 2, Lunar: Silver Star Story, Lunar 2: Eternal Blue, and, of course, Disgaea: Hour of Darkness.
I bought Disgaea when it first came out for $50. Both Lunar games ran me about $30 each, but I got the second for Christmas a few years back. Suikoden 1 was a birthday present back in 2001 and was pretty cheap on Amazon.com for about $15-20. I got really lucky and found Suikoden 2, complete with booklet and case, for around $15 as well. Unfortunately, the copy I purchased is covered with light scratches. It functions properly and everything but there is always the fear that it will just give out on me one day. I'm not sure what I would do if that happened. I have come close to buying it on EBay more than a few times.