The most expensive LPG I've seen recently is 75.9p, and that's not motorway. Given the cheapest around here is 71.9p (BP Brakkers), don't get hung up on prices being more expensive than you used to get on the M40 
Had you have LPG'd it when first discussed, it would have already paid for itself.
Or get a shitbox Zafira... 
It's miles are too low, I think even if I LPG'd it when I first got it, still maybe now just about to pay it off.
I'd estimate close to £1k all in to convert, £350 odd for front end, £400 for rear? Then always a fair amount of little bits here and there, odds and sods needed. Fuel there and back in coverting, have to be done in Brackley.
Payments in curry and beer tokens for any services rendered, doubt I'd see change of much of from 4 figures.
Just been doing some numbers on spreadsheet, looking at miles covered to work. Taking workings days, minus holidays, work from home, ect, then throw in using the bike. It's looking around 16 months easy to pay it back.
I guess if I LPG'd it this summer, if it had to switch to commuting be a lot easier. Have to be a donut tank job though.