I've not tried the sealer/Por 15 approach to fixing the floats from a "4 screw top" unit and I do agree that $100 plus shipping seems a bit much for an item such as a float. I have been successful in soldering several such floats in the past.
I'm not sure what approach you've been taking with the repairs but my method is to drill a small (1/16 dia) vent hole in the float at the top adjacent to the stem and leave it open while soldering. This allows the internal hot air generated by the soldering to escape. Otherwise the expanding air just blows through the solder as your working. After I've completed with the repair and the unit has cooled, I drop solder the vent hole closed. Next I place the repaired float in the freezer portion of the frig and wait a couple of hours allowing it to cool. Next I draw a basin of hot water in the bathroom sink and submerse the float while looking for bubbles from a potential leak area. If a leak is noted I mark it, open up the sealed vent hole, repair the leaking area by drop soldering it if possible and, once cooled, drop solder the vent hole closed again. I've also repaired multiple cracked floats by wrapping them with .010 thick brass shim stock and soldering along all edges. I've checked the floating height of original unrepaired floats against the most heavily repaired units and I'm estimating the pump internal fluid level wasn't changed by more that a quarter of an inch.
A friend recently returned a float I'd repaired after he had successfully used it for the past ten years because it once again leaked and was partially filled with fuel. I exchanged it with a another repaired unit, which he's using, and I repaired the bad unit a second time. A whole lot cheaper that $100.00.
