Not much you need. Is this just as a normal house/attached garage?
Your house already has 220, so you just need to run it to the garage.
Do you just want to run an outlet for your welder, or a complete sub-panel for your garage?
You will need 2 adjescent spaces in your houses main panel to run a 220 outlet, or feed to a sub panel.
Your welder willl likely only require a 20amp service, but consider running wire size larger for 40-50 amps. The welders do seem to run better when they have larger wire to handle inrush surge, and if you want to run something larger in the future, you'll have the wiring. Nice if you need to back-feed your house panel with a generator....
I wouldn't think it'd cost you a 'fortune' to run a sub panel in the garage, I did a 60 amp panel in my last garage, not much money.