Also not a historian but I think it might help out if there were a few more specifics. The price of a horse would have varied through time, region, and type of horse, so a warhorse in 8th Century England is unlikely to have cost the same as a draught horse in an Italian village 4 centuries later. I don't think comparisons to modern prices would help either, as medieval economics were vastly different and very hard to get precise comparisons on. A resource I've seen used before for this kind of inquiry that might help until a more qualified person responds is this price list.

When and where specifically? This question is impossible to answer broadly. It's kind of like asking, "How expensive is an automobile?"

Basically, it depends on literally everything- the training of the horse, the breeding of the horse, the age, the temperament, the location, the quality, the color, the conformation, etc.

A good horse could be almost obscenely expensive. A mediocre horse was reasonably obtainable. But the real expensive part of horse ownership was, and has always been, the upkeep of the animal over time.

So anyway. Please, be more specific.