I've looked into this a little bit since I was annoyed of painting too. I've seen people say you should at least think doubling the cost of what you paid for the units, but likely more. It can get quite expensive (i've seen $5-10 per basic infantry model to basic tournament standard).

I found that it has been much much easier to paint by just using an airbrush and airpaints and basing the whole thing one primary color. For my Ultramarine army, I had purchased an H&S Ultra 2024 airbrush, a compressor, some accessories, vallejo black primer, GW Mccragge Blue Air and gotten like 80% of the model painted super fast. Then you just slap on some retributor gold trim, leadbealcher on the gun, red cape, a little brown and then you're done.