I spend about $100 on painting the tank/seat and frame.
My boss paid for the spaygun, sandpaper and polishing compound though.
If you're determined, you can paint a tank with a rattlecan and get good result. A friend of mine did a really pro job with just a rattlecan. By determined I mean if you like sanding.
Sand the thing down properly, and shoot some primer on it. A good rattlecan primer is about $12. Sand until it's perfect.
Spray the basecolour. You can have most colours mixed in spraycans. Spray about 5 thin layers of the desired colour. Then let it dry for a week.
You don't have to be a pro to spray a gastank. If you get runs or orangeskin, you just have to do lots more sanding.
Start wetsanding with grit 1000 untill it's completely flat. No high or lowspots. Do the same with 1200 grit, than 1500, then 2000. This will take you a day.
At this point you can put on the decals. After than you clearcoat the tank a few times (make sure the clearcoat doesn't react with the stickers, this happens sometimes so test it first) . How many coats off clearcoat you put on depends on how well you spray it. If the decals are thick, you need a couple of coats extra.
Then you repeat the sanding process, and finish it off with a polishing compound and wax. I used 3m Perfect-it for this, but there may be cheaper options.
A bodyshop can probably do a clearcoat for $100 if they want to. The paint they use is 2k, so it's more durable than the rattlecan stuff.