Dry tops only keep you dry (and warm) if you stay in the boat.
Once you exit the boat, the dry top is pretty useless.
To keep warm out of the boat, you'd need something else. Either a wetsuit (of some kind), dry pants rolled with the dry top (not very popular, it appears), or a dry suit.
You don't say whether you can roll nor do you say whether you are planning on kayaking with other people (who know competent enough to perform rescues).
If you want a good dry top (and breathable), those start at about $250. The ones costing $400 are likly Goretex. Goretex has a good reputation but it's expensive. It appears that an alternative to Goretex, eVent has a good reputation for breathability and is less expensive.
($100 seems too low a price for a -good- dry top.)