It is difficult to quote fixed prices, except for the most standard of jobs. Every job is affected by the amount of preparation required, or obstacles encountered, during the early phases.
I generally charge £120 per day that starts at 0800 on site and generally finishes around 1700, depending upon the length of job and when a natural break occurs. I may vary this too, depending upon the overall size of the job, or other factors decided at the start of the process. When a job takes only a fraction of a working day, I generally make an estimate of how long I will take, agree a set price with you and then do the job, regardess of how long it actually takes.
In the course of this working day, I would expect to be able to hang 4 or 5 doors (including furniture), cut and fit anything up to 350m of skirting, dado, picture rail, fit up to 6-7 kitchen carcases (depending upon complexity). To give an example of roofing is almost impossible, as roof repairs vary considerably and new roofs always have individual issues.
Depending upon the size of the job, I will sometimes take an agreed deposit at the start of a job, with the balance expected upon completion. If the amount of work concerned is relatively small, I will expect payment simply upon completion.