T5 official long stay is £28 for the day.
Purple Parking is coming in at £16.95.
Both involve shuttle bus transfers and the Purple Parking option is further away.
Hatton Cross station NCP is £13 for the day. You'll need to incorporate the cost of the tube as well.
Have you thought about a visit to the BA Heritage Centre on Tuesday? When I went they allowed me to park over near Hatton Cross in a staff car park and provided a pass for the BA staff shuttle bus to/from Waterside that stopped at T5 in each direction.