Unless you are looking for a competition standard gas-flowed big-valve head, that price is ludicrous.
First of all, check out the Mini Spares price for a set of cast-iron guides, a set of valves, inlet & exhaust and a set of new valve springs.
Then measure the volume of the combustion chambers on the head as it is now, with valves installed.
Strip the head, including the guides and clean up the ports and chambers. Whether you do or get done some light gas-flowing is up to you.
If some opening up of the chambers has been done, re-measure the chamber volumes, if not use the original volumes as measured.
Determine what the volume should be for the compression ratio you would like. I would suggest between 9.5 :1 and 9.75:1. The formula for doing this is on this site if you search. With the head absolutely level, use a syringe to put in the volume you have calculated and measure how far that level is from the face of the head. This is the amount to be skimmed from the face of the head.
Take the head to a good machine shop and have the valve seats re-cut and the head face skimmed. The machining should cost around £150 give or take a bit.
Lap in the valves yourself and re-assemble. Then fit it and fire it up. With everything, including a new head gasket, it should cost no more than around £350 to £450 which is for the parts and machining. Of course, you will be doing a lot of it yourself which keeps the cost down. Any help you need is available from un nice guys on here .