It costs no more money to drive the car from 1/2 tank to 1/4 tank, than it does to drive it from 1/4 tank to empty. Except that running towards empty has these issues ...
1) stranding you at an inconvenient time
2) possibly frying the fuel pump, because the fuel cools the pump
3) likely causing a service call
Money is tight; you mentioned that and we're not making light of your situation. But seriously, it does not cost any more money to drive it with more fuel on board. Yes - you will have to pay to get it up to 1/2 tank, but once there, your "operational costs" are simply a factor of how far you drive and how agressive or easy you are on the pedal. In theory, you're hauling perhaps another 70 lbs of fuel and that does in a super-tiny fractional manner make a different in fuel economy, but it's truly a pittance.
Once the light comes on, put fuel in it. The risks far outweigh your perceived benefit of squeezing the last drop from the tank. The ONLY time this would make sense is if you were going to sell or trade it, and you wanted the vaule of every last drop of gas. Presuming you're keeping the car, you are much better off just staying out of the yellow-fuel-light range.