Bit like asking how long is a piece of string mate. If your first selection is a lot better (in your opinion) than the second one you might feel a low price is worth a bet. I'll bet odds on if my first selection is streets ahead of any other horse. If your first three horses are pretty close you would be mad to take short odds - see what I mean?
I'm talking from a rating every horse method here, if you select in some other way I suppose some other way of pricing things may apply.

KV