'Avin a laugh - usually a consulting firm's contract will be for agreed daily rates plus expenses (either charged at cost or as a supplemental charge that's a % of the day rate that's been agreed).
Clearly a strategy consultant being billed out to clients at £400,000 a year is a different proposition to a consultant who's being charged to clients at only £150,000 a year. For the former, flying business class doesn't have a material impact on the cost of engaging that consultant; for the latter it would have a more material impact. Add to that the fact that strategy consultants tend to work in teams of 4-10 whereas other types of consultants can be in teams of 50-100 on an assignment... and you can then see that the economics of flying consultants around business class does vary quite considerably depending on the type of consultant you're referring to.
Ger's point - which I therefore agree with - is that it's in the consulting areas where margins per consultant are thin that consulting firms are unable to fly their consultants in business class; and sometimes may even resort to Easyjet!
In my strategy consulting days it was always the case that you flew business class if working on a billable assignment - and in economy if you were on training or on a tender. On client work we always had an agreed client budget for our travel and hotel accommodation and our task was to remain within that budget. The budget was miniscule compared with the actual day rate costs of the consultants, so cutting corners on travel was just counter-productive. Getting stuck somewhere because you had a non-changeable economy seat cost the client £1,500 a day in fees - and saved them only a couple of hundred in airfare!
Having said this, would certainly agree that the regularity with which you can be based at home is without question the greater prize to secure. Champagne doesn't taste so sweet when you find you're still at Frankfurt airport on a Friday night while all your friends are out on the town!