It depends on the game! Blackwell Unbound took four months and about 800 bucks, but I was deliberately making it on-the-cheap and it shows (the artist was a college kid on her summer break!). Blackwell Deception took a little under a year, and cost about six thousand, not including the marketing/PR expenses (which would bump it a few thousand higher).
Gemini Rue took Josh about 3-4 years to make, but he did all the art himself so it didn't really cost him anything but time.