20/60/30 was most economical I found. Sticky globs that quickly destroy everything (except spiders), you can take out 6-8 things if you get the spread correct. I ended up with ~$20k spare at Lv16, with two upgrades.
The reason for multiple slots is so you can have secondary weapons (long reloads) and passive ones. If you buy 4 heavy cannons, of course you're going to get your ass handed to you. You want one main weapon (Double machine gun is good), 1-2 secondary long reloads for different situations, I use double snipers for taking down fast moving enemies and enemies with -damage mods. And then 0-1 passive, depending on the levels. Decoys are pretty good for that purpose.
Overall, good game. A lot of effort seems to have gone in to it.
dotSeed's comments