How do you determine how much to spend on speakers

A chime in note on cables. If you haven't already broken the bank on speakers and amps, always ALWAYS get rid of the stock power and interconnect cables that may come with your equipment - especially where the analog signal path is concerned. You can get low-end hi-fi (still very decent) for $50-100 which gives you a good bang for the buck.

The nice thing about cables is you can do it gradually over time as you feel the need. I suggest starting with your amp since it's the most power hungry component. Better the signal path with quality interconnects next and then the power cords to the rest of your components. Finally, add a conditioner if you can - even simple ones will do wonders for imaging.

Adding it up, I spent about 10% of my system cost (~$11,000) on cables (60% of that was speaker cable). $100 Transparent Audio power cables (3) and interconnects (1) have done a lot for me. I've tried some more expensive stuff, but found better use for the money in a power conditioner.

A really good 8 ohm speaker could dip down to 6-4 ohms. A good 4 ohm speaker can go down to 2-.2 ohms. If your amp doesn't "double down" when resistance is halved , you're losing music. See what your amp can do , and buy speakers appropriately . Or buy an amp that will handle anything you throw at it and pick up some Maggies.

"How do you determine how much to spend on speakers "
More than you want too :-)

Based on my experience, I would definitely start the development of a system with the speakers. It is the speaker that mostly characterizes the sonic signature of your system. That is, electronics can change how well a speaker sounds, but they don't change the speakers' basic sonic signature. So, i would first make sure that I get a speaker that has a sonic signature I like. I would then try to optimize the electronics based on the speaker I've chosen. I don't think power cables make a huge difference. Interconnects and speaker cables, on the other hand, can make an enormous difference. The above probably suggests that the speaker is the hub of the system, something that you don't want to change that frequently because the rest of the system needs to be optimized on the speaker. For this reason, I would over-invest on it. How much depends on the size of your room. Smaller rooms afford you a smaller investment. I just bought a pair of reference 3a Decapo i for my smaller room...I invested $1200, and I believe it's hard to find much better speakers for a very small environment. Of course medium and large rooms require more expensive equipment...but even here, you can reach pretty good results with a $3K investment. If you like the midrange, for instance, Piquet modified ESL 57 is very close to the best...and they cost $3K.

My logic would be,A SPEAKERS and B AMP are the most important starting point.The rest in time.

Good Luck and have fun!