If you have the resources, salvage in the field is cheaper than salvaging the Mechs after the mission is over. Any Mech you salvage in the mission with the salvage craft, you keep with the rest of your Mechs in recovery. It only costs 10K resource points in the mission to salvage a Mech, while after the mission in the salvage screen it can cost from 20K to over 150K resource points for salvage. However, only do this with Mechs that are worth salvaging. If you have any resources remaining after all the objectives are complete or the enemy destroyed (whichever comes first), salvage whatever Mechs you can and sell the useless ones when you get back to the screen.