cc117 - It depends on whether you are really that desperate to move I guess whether it's a good idea to clear your mortgage through the sale!
You say rent is more expensive than your mortgage.....but don't forget you are only paying the interest on your mortgage currently which is doing nothing to bring down the capital amount.
I presume you want to move to the area where you saw the house for 99k? Is that where you have been pricing rent options?
Could you rent in a cheaper location while you are saving the deposit for the 99k house?