Wetsuits require multiple wetsuits for even small temperature changes which increases costs. I have a trilam and can change layers to account for various temperatures,from tropical to ice diving. Yes, undergarments can be expensive but they can also be very cheap, if you go the REI or Army surplus route. If you're not average sized, you have to order a custom wetsuit. Anybody price those lately? A good one can be as much as $800, not counting accoutrements!
Yes, as, my trilam was expensive and custom but I could have gone the Fusion route. Plus, drysuits usually have a longer lifespan than wetsuits so that also decreases the cost per dive.
Finally, as TSandM was saying about the DCS risk. I watched a DAN video presentation by Neil Pollack, probably very similar information to what she's referring to. If you start the dive with your heater on and it quits, even a very ordinary, recreational profile can carry the risk of DCS. It was very sobering to see the statistics of how many healthy Navy divers got DCS when they started a dive warm and ended the dive wet, even when they dove well within dive table limits.