Depends on a whole lot of things. Where you at environment wise, if you don't mind me asking?. Up at elevation above 7000, you have much different considerations than around 5280 in Denver. How much height do you want to give your plants? If you want to keep them lower than 5-6 feet, you can probably get away with PVC framing. If you want to go higher, you're looking at galvanized steel.
For $600 or under, I suggest getting a few 1/2" x 4' lengths of rebar. For a 10 x 20, probably 12 of those.
($4.35 each from Home Depot. So a little over $50 in metal.)
Get your footprint set up. Pound the rebar 2' into the ground from the corners and around the perimeter every 4'
Grab some 3/4" x 20' pvc lengths. These should be about $8.00 each. You will want 6 arches, and maybe a couple extra for trussing for a 20 ft. high tunnel. So probably 8 or so lengths for $60 or so in PVC.)
Slide these over the rebar, creating arches across the ten foot width. Tie a couple down the length of the arch system, to truss it all together. Make sure these run down the inside of the arch, so it doesn't affect the plastic.
Grab some 10 or 12 mil plastic like this option:
https://www.ebay.com/i/252645003637?chn=ps&var=551560738404
That's more than you need though, so maybe you can find a more economical option. In any case, that is about $200 shipped.
Grab a couple 20' treated 2x6 lengths of lumber from your local building supplier. These you can run along the bottom length as baseboards, and gives the plastic something to stretch and tack into. If you just have a truck, or no lumber delivery, you can just grab stud length boards and make it work. I forget, but I think a treated 20' length will be roughly $25 each.
So there you go, basic structure for ~$350. Then you have $250 for whatever accessories you might want to add in, fans, supplemental lighting, fasteners... whatever you'd need to round her out.
Doubt you could go cheaper and have something that will stay up all season through CO winds.
