The ones from the first Botswana special (minus Oliver) are still there and were shown in One for the road, the wagons they used in Rwanda (Volvo, BMW and Subaru) are still there as well, most of the other cars from both TG and TGT were either sold to some locals for cheap just to get rid of them or destroyed for various reasons.

A few notable exceptions include the Bentley used in Madagascar that made a brief cameo in the Farm show and was then sent back to Bentley for their museum. The two Polar Hilux pickups used in the North Pole special have also been preserved and sitting in a museum*, and a couple of others.

To my knowledge, none of the cars used for any challenge (in specials or otherwise) have ever been sold to private collectors, other than the MGB GT (which they sold to Hammond) and the few from TGT that have been documented on Drivetribe.

Most of the "reasonably priced car(s)" were either scrapped (most of the Lianas) or sent back to the manufacturer. Like the Kia, for example, they had three in total over the years and they were all press cars never owned by Top Gear, just on loan from a dealership.

Technically, the dragster Hammond almost died in has exchanged owners a couple of times and has since been restored (Hammond sat in it again a couple of years ago).

*Fun fact, the Hilux May and Clarkson technically drove across the physical North Pole was the backup vehicle because thte original (seen in most shots) broke down.