If you are planning to grow vegetables on a patio and are not too concerned with the appearance of the pots a great cheap option is florists buckets.
The buckets below are available at Morrisons and come in 3 sizes:
I have used my supply of buckets for:
Tomatoes (try cutting the plastic base out if using a ring culture system)
Peas (for pods and pea shoots)
Mint and other herbs