What's the best greenhouse for 300 to 400 dollars?
4 x 16’ cattle panels built on to a 2 foot elevation. I built mine it’s 30 foot for under 500 bucks. 6 mill greenhouse plastic from Amazon for $118.  and I’m including that in the $500 cost.
Build your own with materials that you can cobble together. Can’t buy anything quality for that budget.
You can purchase a 6 x 8 at harbour frieght right now 50 bucks off. I had one at my last home lasted me 3 years. To my knowledge its still in use I move 3 years ago. I think it was worth the money.
If you can do a little carpentry 2x4s,
plastic
Staples and 3-in screws. $100 for 50x100 clear plastic Staples and screws. The rest you could spend on 2x4s. I put a single coat of primer on the 2x4s to make them last longer and reflect light. I built an 8x8x8 cube with a 10° tilt on the roof. My studs and rafters were spaced 32 in oc. The plastic on the Sunny side only lasts a year. I've heard you can get UV resistant plastic but not at my home Depot.
plastic
Staples and 3-in screws. $100 for 50x100 clear plastic Staples and screws. The rest you could spend on 2x4s. I put a single coat of primer on the 2x4s to make them last longer and reflect light. I built an 8x8x8 cube with a 10° tilt on the roof. My studs and rafters were spaced 32 in oc. The plastic on the Sunny side only lasts a year. I've heard you can get UV resistant plastic but not at my home Depot.
Keep a close eye on the marketplace. I was able to pick up 30 hoops for 10 bucks each. Sometimes farmers are happy to sell cheap if you come and take it down yourself. Farm plastic supply sells the poly for a decent price. Buy just the amount you need. Get the plastic a little wider so you can bury it on either side. Dig a small trench outside of the stakes, lay down the plastic flat and throw on a few inches of dirt. Repeat on the other side, you can get it nice and tight. For the end walls, you could let the plastic go long and just figure a way to close it. This is on the cheap so you will need to get creative. Good luck. It can be done!