That does sound rather excessive but after having a bit of a look in TIS there is a fair amount of work involved removing and refitting the heads, but that doesn’t justify the $8,000 or $13,000 quote.
It also appears these heads don’t have replaceable valve guides as far as BMW is concerned, although I note there are aftermarket valve guides available for the M62 & N62.

BMW use oversize valves (larger diameter stems) along with reaming out the valve guide to fit. This adds additional cost as you need to then purchase new valves of which there are 32 if you replace all of them plus the cost of reaming all the guides and re-cutting all the seats.

In my view, replacing just valve stem seals is a waste of money in most cases and will probably only be a temporary repair at best. There is usually a reason why the seals need replacing and that’s normally because the guides are worn.

If you intend keeping the car, I would look at taking it to a reputable independent shop with a view to getting aftermarket valve guides fitted. You will probably find these last longer and will be cheaper to replace the next time around.