Should I focus on buying mods or farming them from battles?

Nice post @Vendi1983 but a couple things I do different:

1) I level most grey to 6 because the cost is very small, if I hit a +4 or +5 speed I'll take it to 15 and slice to green. I've ended up with a couple of mods that hit +13-15 this way. Anything that doesn't roll a speed at level six I sell for a profit (albeit a smaller one).

2) After mod farming for a long time, I have all the speed arrows I can use plus a bunch more. I sell those too now.