How do you determine how much to spend on speakers

Make that the Pathos Logos.

The point here is to select stuff that is of a uniform quality especially if you are on a budget. You can do extremely well on so called budget items. The majority of problems occur with the ROOM and NOT the equipment. I suggest the OPPO players for CD and SACD. An integrated amp from NAD and Kimble Hero interconnects. Speaker wire should be Spitz anti- cable and whatever is left should be spent on the speakers. If you have some money to spare I would try and deaden the listening room.

Find the best sounding speakers you can, find a way to talk whoever owns them down to what you can scrape together. Then figure out what made them sound so good to you ask your self is this what I want to go with my speakers and can I in the near future buy this amp cables front end or is what I have close to or as good as what I heard. When at a dealers shop make notes of what the speakers you like are being driven by cables amps front end and anything in the signal path also note room size treatments tweeks it all counts the closer it is to what you have the better the chance it will sound that way for you at home. That is if you can't do a home demo. In my case I spend more on turntables and speakers as I find I keep them longer than cables or amps but that is just my way. Try and remember this is a hobby not the search for the holy grail have some fun with it. As to the how much to spend "I will go all in with all the cash I can lay hands on if I find a speaker I want" Martin logan CLX this last time I even bought the amp cables that were driving them.

I actually think that it is a non-linear function where more you spend on the system on aggregate, less critical it becomes as to what percent of your total budget should be spent on the speakers. Put another way, if someone's budget is $3000 for the whole system, I would say, spend at least 2/3rds on the speakers. If your budget is $8000, then it is a totally different relationship and it will depend more on your priorities (efficiency, brightness, dynamic drive, low-end, mid-range, resolution, even details like off-axis response).
My two cents.

Are you interested in good musical qualities or good sound qualities??
I've had expensive speakers ($8800 Cremona's) and now have $1200 speakers (Magnepan MG12's).
If you are going to spend a lot (like more than $2500) you'd better have the really solid (and dead sounding) walls of a great listening room to go with them or you will still have crappy midrange and treble. I've had a pair of Proac D15's that were beautiful sounding in a high-end apartment but sounded awful in a house I later bought (until I covered all the walls with persian rugs to shut them the heck up).
You can get by just fine with el cheapo MG12's or mini monitors if you have real high end source and amplification components though. It's all about the music, not the sound. Speakers (IMO) don't seem to be as much about the music than they are about the sound. One of the best systems I've ever had was a pair of $875 minimonitors (linn tukans) but they had a $2500 source (LP12) and $7000 preamp and power amp (Kairn/Klout) in front of them.