Your friends should respect you enough for your knowledge in the trade not to question you and should also realize that as a friend you're giving him a reasonable price not ripping him off,just because he's a freind it doesn't make bread and butter any cheaper.If he doesn't understand the time issue of prep work,take the price you have in mind for two days work divide it by 16 hours then tell him"okay,we'll do it this way instead,forget the $400:00 for two days work,my hourly wage is $35:00 an hour.That way 35x16=$560:00 so you come out ahead and he thinks he's paying you by the hour not the job.Or say,"my hourly rate is $35:00 per hour ,but since you're a friend I'll do it for $25:00 that way it's the same price as the 2 day job rate plus if you happen to take longer you still get your extra money.