The rule of thumb here in the U.K. is to think of a (large) number and times it by the number of days until your birthday! Typicaly you would be loking at four digits in £ stirling!
The work woul entail digging a trench from the nearest suply feed, running a cable (20mm split concentric) jointing one end and terminating the other in a custom built (by you) brick shed via a 100 amp cutout and meter! Don't even ask "what if it is overhead line fed"!Then you need to arrange for a supplier to take you on. Don't forget you are not a source of rvenue to these guys! Unless you are going to be visiting the same site anualy this is NOT AN OPTION! Sorry I forgot to mention that you still need to have your instilation checked (anualy) to ensure it complies to the I.E.E.Regulations, for which you need to be NJIC/NIC approved (a typical house check is about £100). KISS (Keep It Simple Stupid!) Use a generator with your sound guy's own kit plugged in! It should be coverd by a Portable Appliance Test (sticker on the plug)! If he/she is a professional they will have their own insurance as well!
Sorry to winge on but this stuff is the only thing keeping you out of Jail if somebody gets hurt..... not that they will