There is currently no way to see the stats you are losing other than equipping both and comparing.

In the past, optimal level stats were pretty scaled up to the correct level, but now it is very restrictive, ie you will get absolutely no stats until about two levels below, and even then a level 8 weapon will be better than a level 35 for say, a 33 Lancer.

People are also very silly with the markets. Try looking up recipes at mooglebox.com and you may find the ingredients total 1/10th of the market price, to have a friend help you out.