Are we talking about trading an iPhone 4 for $300 cash + SE mini X10?
I dunno, that's not THAT bad of a deal. Looks like iPhone 4's sell for about $500 all day on eBay
mini X10s are about ~$200, that's about $500 "value" right there, assuming the X10 is in great condition and you turn right around and sell it.
Better yet, tell your friend to sell the X10 and give you $500 straight up. If you were to sell the iPhone straight out on eBay or something, you could probably get a bit more, but it looks like $500 is a fair/average price for it.