The other option is to find a beater 50/2 AI. Take the aperture ring and the lens mount assembly, complete with rear element baffle (if the AI lens has five mount screws, otherwise, only the rear element baffle), and mount those on your pre-AI K version 50/2. Turns your pre-AI lens into an actual AI lens, except for the focusing ring grip and the serial number. Quite a few of the K-version Nikkors can be AI'd in this manner, making a more-complete AI conversion than what you can get anywhere else.
That said, if the AI conversion is being done by the guy who I think it is (microbee on eBay), you'll be pretty satisfied with his work. Had him modify my 28/3.5 so it can be used on my F4 without breaking said camera.
Re: 50/1.8 Nikkor prices, one of the local shops here in the Seattle area has a pretty nice long-nose AIS version for $89.50. Much less than what you're expecting to pay for one.