O'neill makes a great dry suit for about $350 US.
Bottom is like a wet suit (but no water) and top is like a dry suit. You layer underneath for whatever conditions you are in. The big thing is that there is no constriction on the arms, hands or upper body. You should get them a little large to allow layering under the bottom half too.
One of the guys in our area got one and within a year, 8-10 of us are now using them. We are in Dallas, Texas, but sail all year long. Generally, if it is sunny and the air is in the 40's F (7 c) or above, we have no problem. The lake water at its coldest is usually in the 40's F (7 c). Most of us have booties, hoods and gloves, but the hands are usually the problem. Warm gloves are bulky and have no feel on the boom, while the thinner gloves with good feel are cold.
I should also add that I have been using semi-dry wet suits for the last 20+ years, usually 4/3 mm. They are fine, but generally too tight on the arms. The last one was a Neil Pryde with a pullover head flap that made the short zipper pretty much waterproof. The O'neill is MUCH more comfortable. I have had a little leakage around the neck and a little at the leg, but there is more sweat than leaking water in the O'neill.