A decent mig is a big purchase up front and gas can get expensive if you do a lot of welding.
I just got my (admittedly large) gas bottle refilled today, cost me $103, I also pay bottle rental fees which come to about $17 a month ($200+a year) For me it is worth it, I use my welder a lot and I have the machine, skills and experience to get the best out of it, but for some this is just too much of an outlay up front and may prevent them from taking up welding/building bikes/whatever as a hobby.
What the answer is I dont know, I would always suggest going to a welding class or getting a competent friend to teach you, I also think that people unreasonably dismiss the humble stick (electrode) welder for building bikes, if you have some training you can build any of the bikes on here with a cheaper but still decent little stick welder, it will require more patience and more time, but, less moving parts to go wrong, no gas to worry about and a decent stick welder is probably half the price of a decent mig! Go to the welding course or friend and try it, you might be surprised!
Just my 2 cent opinion... By the way, all this is coming from a fully certified structural fabricator and welder...
Luke.