It's not that expensive when you consider that England have been rollicking along at 4 an over in this series, and that Australia regularly score at 4.5 an over. It's certainly not, of itself, grounds for not selecting him, and is more than counter-balanced by his peerless strike rate of recent summers.
I just think that, as I eluded to above, people are really over-doing this 'Jekyll and Hyde' thing with Tait. That sort of branding is just the easy, lazy option which sounds very interesting, but has little basis in fact- particularly in his most recent, and as yet his only full, season of FC cricket.
2004-05: 62 wickets @ 20. Economy rate of 3.35
3.35 isn't exactly bleeding runs.
Look at the whole package, not just one aspect of things, not that the aspect you've highlighted (his economy rate) is that damaging to his cause anyway.