Here is my DIY auto top off set-up:
I bought one of the 5g plastic water storage jugs from Wal-Mart. (You need to make sure that it is not one of the flimsy clear jugs if you want to store limewater in it, I found out the hard way the limewater eats through those!!)
I took a piece of hard airline tubing and put it through the vent hole to used as a "pick up" tube.
I then ran airline tubing from the pick up tube to an Aqualifter pump.
I then ran airline tubing from the pump to my sump. Note: the end of the airling tubing in the sump needs to be higher than the water level in the storage container or you will get a siphon, for the same reason, the end needs to be above the water level in the sump.
The Aqualifter pump is controlled by a float switch in my sump.
I probably have less than $25 in this set-up. It has been running for about a year now with absolutely no issues. I do make sure to run vinegar through the tubing and pump now and then to clean out any calcium build-up.
Here are a couple pics. I will try to get some more detailed ones if necessary.