If it was me, I would work out roughly how much you can get done in a night shift (you should know how much you can clean normally and then add on some hours for the gum depending on your machine set up)
Work out diesel, for the hot box and machine, diesel for the van, wages etc.. I'm sure you know all this.
Charge a day rate. You might not get as much done in the first couple of nights, but you will soon get faster, and build up a technique that suits the job.
No way would i be charging per m2 for this type of work. I don't even charge per m2 for driveways.
Speak to Lee at Blastaway, he is quite good at the old town centres