Harbor freight in North Raleigh. Their 6'x8' greenhouse is on sale for $299 online. They should honor the price in the store. You should also look for a coupon code online, as you can usually get an additional 20-25% off that sale price.
Be aware that the harbor freight greenhouses are bottom of the line hobby greenhouses. They've been reported to be flimsy and difficult to put together. However, they're also probably the most commonly purchased greenhouse, due to the price. You can find a lot of resources online for strengthening the construction of the greenhouse and installing your own shelves or benches. I can very close to purchasing one this season but deciding against it, as I don't need any more projects to work on at the moment.
If you are looking for something smaller or less permanent, check big lots. They have some for under $50.
Harbor Freight Greenhouse