It's silly to compare black and white to Velvia. For that matter I don't shoot color in 4x5 except when I get a deal on expired or partially used boxes of film. I shoot it with a rollfilm back if I see something that has to be in color, assuming one of my lenses will frame it on 6x7cm. 4x5 color is just too expensive, but if I did much of it I'd process it myself too.
Regular black and white 4x5 is all over the map depending on brand but using Ilford (cheaper than Kodak, more expensive and much better QC than Foma) as the comparison it's about $1.20 to $1.30 a sheet from B&H. That's not too bad. A few cents more for developer and fixer gets a negative. Include the proof - I cut four 4x5s from an 8x10 sheet and print proofs on RC paper, $68.95/100 from B&H. Seventeen and a quarter (rounding) cents for a 4x5. Even assuming I used two sheets including tests (I don't usually) that's $0.34, add a few cents for paper chems. The entire negative plus proof comes out to less than two bucks, easily. Three times as much for the instant is not something I will use much of. Not saying I couldn't, it just wouldn't be worth it to me. For exposure proofing I can continue to use the color Fuji 3.25x4.25 film at a buck a shot or so.