Build your own grenades.
Give resources to a Weaponsmith, they give you schems, you make subcomponents in an Equipment factory, you give those to Weaponsmith again, get final grenade schem, crank out grenades in your factory.
Have 2 factories, or for the most efficient means of cranking them out, 3 factories - use 1 to build the first subcomponent. When it's done with the 1000, put them into the input hopper. Start the other 2 at the same time. As they finish subcomponents, put them into the input hopper of the first factory. The first factory will be cranking out TONS of grenades before you know it.
The actual cost of resources for 1000 grenades is a LOT less than buying them retail.
They are kind of annoying to make in serious quantities, which is why WS's charge quite a bit for them. That, and the fact that FS grinders are willing to pay a lot just to get a few crates.
Anyways, trust me. Spend some cash on a couple of harvesters, BER10's are the most efficient cost-wise (but BER13's dig up more/hold more), and pull up the resources yourself. You'll save millions of credits.
Only buy grenades retail when you've got to finish up a buff session and you just need an extra couple of crates. Otherwise you're spending way too much on grenades.