There are a few shops that sell wood near the old city area the prices seem quite high. I am thinking about building a small deck (3 x 4 meters) but probably won't bother if it's too expensive. Are there any better deals to be had if you travel outside of the city?

Try a search of the forum. This has been posted many times in recent years.

We recently went through the same thing when we built our deck. Looked around for wood that was dense enough to resist termites and found out it was way too expensive. We also discovered there's an artificial wood (Conwood?) that looks real, bug proof and weather resistant. Cheaper too. Comes in standard plank/board sizes and cuts and drills the same as real wood. We bought ours from a shop on the Chiang Mai-Hong Dong highway about 3/4 the way down to Chom Thong on the left (southbound) side. We painted a couple coats of stain/sealer on it and it beads up the rain and no termites so we're happy with it. Sorry I don't know the name of the shop but imagine other wood suppliers around may have this stuff.