Dexters are ok if you cross them with something commercial and get a saleable calf.
I've just got £919 for a Dexter x Hereford bull, at 21 months and weighing 550kg.
You wouldn't get half that for a pure dexter bull.
If you keep them pure you get next to nothing for them unless they are of superb breeding, and even then the market is very limited, because you can still get registered ones of unheard of breeding very cheap.
The last ones I bought at market cost me between £55 and £90 and they were all decent, pedigree cows. I put them to my Hereford bull and the resultant calves were worth up to 3x what I paid for the mothers before they were even weaned. (Obviously I didn't sell them at that age, as they were worth considerably more fattened.)
Dexters are a smallholders' cow because they are small and cheap, and you can fit a whole one in a single freezer. But there's a limit to what you can eat yourself or sell to friends.
They are a useful animal to learn with, but you'll soon move on to something more profitable.