If you watch Ebay or Craigslist enough, you start to be a pretty good judge of what things sell for, on average. But these are pretty fluky markets. One day you'll see a bike sell for way more than you'd have expected; the next, no one bids on a perfectly good ride. So, unless the two of you have other willing buyers lined up, I'd say the bikes are worth whatever the two of you say they're worth. If it's a win-win in terms of the sizing, I'd say even trade makes sense. If you are benefitting from the trade and he's not, I think a reasonable person would conclude that you owe him a little something more.