As others have mentioned there are alot of brand names out there like Bessey, Wilton, etc that make excellent clamps but, like you said, they are expensive. As someone who does alot of relatively heavy fabricating, my advice is this. As time goes on look for the the older name brand clamps cheap in pawn shops, flea markets, etc. While it's true good tools are nice to have, what most guys on here seem to forget is that it more than likely they didn't start out with the most expensive, best grade tools, and it took them many, many years to come up with the tools they have now. Now if it's one, critical tool it's often possible and advisable to buy the best you can get. Unfortunately in an instance like this, when you need multiples of one type tool, trying to go 'first class' from the word go is pretty much impossible because of the high cost.
So, until you get some years, and alot of pawn shops and flea markets under your belt, check out Travers Tools site. Their TTC brand clamps are sufficient for the kinds of work you are doing, and are reasonably priced. I've got several of them that I've had for about 8 years and have had absolutely no problems with them holding anything I needed to hold with them....within reason. By that I mean the ones I've got aren't made to hold down something while using a 5 foot cheater bar to break a nut loose, BTDT, as they will bend just like any of the expensive name brand ones. In the case of the one I tweaked I simply put it in the vice and twisted it back into shape, no problem. My point is you can abuse any tool, regardless of the name brand, and make it fail. In the case of clamps there really are junk ones that are a waste of moneu, good ones that are nice for someone like yourself just starting out on a budget, and the expensive, better ones for thise with the money and/or ctitical need to have the absolute best. Even then if you try to use a 12 inch, light weight clamp where a 24 inch heavy duty one is needed, it's not going to provide adequate service, regardless of the name brand.....
That said, having built truck bodies for a number of years, I acquired about a dozen pairs of the Vice Grip welding pliers like pictured in one of the other replies. Having several of the ones with about a 4 inch throat, and at least one pair with say a 12 inch or more throat is just about a necessity when working alone. Even if you intend to use a C clamp on something it's always easier to hold pieces together and use one hand to clamp them with a welding clamp for the intial clamp. Then, as long as your not at the limits of the clamp they will hold the joint just as tight as any c clamp would.
Good luck, and I hope this helps you out some.