Material, bits and software software can be a biggie some of the best stuff is not cheap at all. Bits last a long time on wood maybe 30-50 cuts before getting dull, I don't know about other materials. Wood, I get the best stuff at Home depot in the select boards to avoid warpage after the cut and that oak can be as much as 50 bucks a board! I usually wont charge more than 20 bucks for projects I made on my own now if they want me to design them something that might be more depending on the job but not if its just a cut of a photo. I don't have anything left it all sold.
I'm not trying to get rich or support myself off of a hobby just help with the cost.
To TXDave.......first suggestion is to not give anything away. You're family and friends are likely to be the first ones to want you to do something for them - at least make your cost for the wood and a little for the bits back on every piece. If you do it for free, they'll expect it for free. If you charge, they'll give you references to other people and tell them that your prices are reasonable. Then you begin upping the price till you are actually making money on each project.
Don't cut too deep on each pass, you put a side strain on your spindle bearings and there have been a lot of problems reported with spindle motors going bad. If you are using gimp or some similar software, use a nice border around the entire piece to make the transition from a flat surface down to cut depths - it makes the piece look better. Warped or cupped wood can give you some problems, so store your wood flat - or some have suggested putting it under a weight for a few days before cutting. If you do a cut through, like my avatar, place a sacrificial board under the piece board to insure you don't cut into the table. The work piece can be held in place on the sacrificial board with double edge scotch tape for the cutting.
I haven't tried multiple passes, but most people on this forum report not having much luck with it. I think that it doesn't line up just right for the second pass. There are a lot of good suggestions on this board, and a couple guys to watch are Bob and Greg who have a ton of experience with this.
Cut across the grain rather than with it. Always run the "border" mode to make sure your piece is oriented right - I'd of made some major mistakes if I hadn't done this.