I think you could automate it with a future value calculation included.

For example, have one half of a players "trade value" be his current value, the other half future value. Future value might be based on projections for the next five year - assume players will improve by X% each year until they are age 26, stay stagnent from 27-30, and start to decline by -X% after 31. Assign generic values to draft picks by round.

It would be crude, but it might work well enough to help flag multiplayer trades that were not as obviously out of balance...

Fred