Since i've had ny boat i've replaced the batteries and all the wiring between the alternator, batteries and main fuse board, plus installed new cables for the fridge. Based on my experiences I would guess that to install a shoreline, charger, inverter and 230V wiring would probably take 2-3 days. Multiply that by a professional labour charge (say £30 per hour?) and add the cost of the equipment, cables etc and you'll have a rough idea, but the only way to find out is to ask an electrician, but before you do that you need to be sure what you need done!
As others have mentioned, I would query what you need 800W of 230V power for? Also consider that to run an 800W inverter at full power for an hour will use 66.6 Amp-hours. Given that you can never discharge batteries entirely you are probably looking at needing something like one 100Ah battery per hour, and then you need to think how you are going to recharge them.
I've had a boat for just over 4 years, using it for weekends and holidays, and I don't think i've ever used more than about 100W
Peter