You might can find some neoprene waders or wetsuit for that price range. Drysuit will be a bit over budget. You can get some good deals on drysuits on ebay
I hate dry suits and really prefer wetsuits...no maintenance and you can pee. Unless the water is below 30 degrees I can stay in the water for 2-2.5 hrs in a 5/4 wetsuit...the dry suit is warmer of course but there is maintenance and they are kind of a pain imo. Never wear anything that can fill with water in a river/bay/ocean like chest waders. I have probably heard of a dozen or so people dying clamming and such in my lifetime.
You may want to go neoprene waders, wear a belt and PFD..I wore waders for several years, stepped in my own holes.. fallen into and all was fine But you have to chose the days you wear them.. calmer water., wear the belt, and PFD at all times. The Bay is forgiving since you can walk out 100 yards and still be waist deep. The Ocean, a different animal....
I have drysuits , with dry gloves and thermo boots... just the glove system to keep my hands warm is over the 200 mark.
And they do make P Valves for drysuits...
And that is a Excalibur endcap, it just never made the cut to be a endcap
Adam B you must be young.. and a surfer at one time.. the most the boys up here can do is 40 degrees with the 7mm farmer johns, and jackets...