I am in the fortunate position of now being able to say "I fly Business Class in Europe or First on inter-continental. When told that they won't pay for it, my response "You need my expertise more than I need your money. Goodbye".
About 50% of the time, they knuckle under and pay. Admittedly, they then get a very cheap rate per day to keep me out of high rate tax. The alternative is a very high rate per day but I absorb expenses.... which still keeps me on the edge of high rate tax.
Being that I don't need the money, in those famous words "I don't give a damn.."