When we built our new home in 2012-2013, we went with a sealed/conditioned crawl space. It's pretty much the standard in this area now, and my understanding is that it has become a "best practice" in the humid southeast US. So far I am very impressed with it. The crawl is dry and clean, and there are none of the issues you'd have with a vented crawl in this part of the country.
One corner of our home has a 7' tall foundation height, enough for a full size door. We poured a slab down in that corner, and that's where the hot water heater, well tank, and water treatment tank are located. It's also a great storage space, so I have some shelving down there. Because the area is conditioned, we store furniture and other household items down there, which has been awesome. Could never do that in a vented crawl. From that tall corner, the foundation tapers to about 2' height at the other extreme. In between the two corners, I have all sorts of stuff stashed, including spare lumber, a kayak, etc. I'd say it's like having 60% of a basement, and really helps me keep other areas of the home neat, but still have my "junk" nearby stored in a clean dry place. Almost like an onsite conditioned warehouse.
The HVAC ducts for the first floor of the home are down in the crawl, and there is one small extra register on the end of a trunk that pumps conditioned air into the crawl space. There was a return duct installed (code requirement), but it is blocked off at the recommendation of our HVAC guy. The idea being to keep a slight positive pressure in the crawl space, and let the return air bleed through all the cracks/gaps/etc between the crawl and first floor only when the blower is running, rather than have a return duct that is always open to the first floor. I imagine the same result could be achieved with some sort of passive damper on a return duct.
Our previous house was built in 2000, and had a regular vented crawl. I can't say I ever had problems with it other than a little mold at one point. But I was careful to check down there periodically and make sure the vapor barrier was in good shape (that was only plastic) and the vents were open in summer. The thing is, other people in the same neighborhood had real problems with their vented crawls, such as dampness, mold, and odors. Some had to install fans and additional venting. I am only guessing, but it's likely those other homes had moisture sources, either due to poor drainage, poor grading around the home, or just bad location.
Even conditioning the crawl in our new home, our heating/cooling costs are about 2/3 of the old home, per square foot (living space only). The new home happens to be 1.5 times bigger in living space, not counting the crawl. If I had to take a stab at factoring in home size as well as crawl space volume, I'd say there is little to no cost to conditioning the crawl, all said and done. Some of that is due to having a more efficient geothermal HVAC system in the new home of course, so exact estimates aren't possible. But suffice it to say that even when conditioning the crawl, we are spending less on HVAC per square foot of living space than the old house.
