Unfortunately, that is a trap that many people here seem to fall into. You always have to look at the total cost of travel, not just the headline cost of the main ticket that you intend to travel on.

The other point which always entertains me is people who are doing TP runs which they think cost £x per TP - but only if you look solely at the cost of the ticket. For some of them, by the time you've added the cost of a hotel, transfers, meals on the ground etc the cost of the TP run is much higher than that. Fine if you were going to have that weekend away (or if you were planning to camp for free, as one FTer did in JER IIRC), but not if you only wanted the TPs.