I came across a few new sets the other day that had great Price per Part ratios
31009 Small Cottage. $19.97 for 221 pieces. That's $0.07 per piece. With plenty of basic bricks in Light Bley and Red you should be able to put this to good use making terrain. It also has a small amount of useful parts for mech building.
31010 Treehouse. $29.97 for 356 pieces. That's $0.08 per piece. And not only do you get a handful of useful mech parts -- plate clips, tile clips, battle droid, arms, etc. -- you get almost 40 1x2 White and Brown bricks. Along with some other bricks in the set this should allow you to build some walls or a decent building for use as terrain.
31012 Family House. $69.97 for 756 pieces. That's $0.09 per piece. Like 31010 it has a handful of fiddly bits. It also has basic bricks in Light & Dark Bley and White in enough quantity to be useful for terrain. It also has a light brick and it's still affordable which is a miracle.
The only thing these three sets have that I don't really like is minifigs, which aren't all that handy when it comes to building mecha. Otherwise they are all excellent deals.