B&Q and Wickes in the UK(and others, probably) have cheap nominal 3x2 timber for studwork. I use 3 pieces glued with Gorilla glue along their narrow edge and clamped to form an 8ft x 9inch plank for a fiver (sterling). I use one for gardening and it has lasted 4 years exposed to the elements full time.

Using 2 pieces glued along the wide edge and clamped will give a very serviceable fence post for about a third of the price of standard 3x3 fence post.