Spend what you can afford on speakers and make sure you get an amp that will power them with ease. Under powered amps will damage the speakers
It's not about cost it's down to what sort of sound you want to achieve. You dont nesaseraly need big speakers for big sound. I.e movies most surrounds systems will do but if you want a cinema room then the cost could easy be in the thousands .
Music in stereo depending on how loud you want it can sound good at both ends of the price tag .
Again it's down to what you can afford and what's the main use.
I survived from 2001 through to 2008 with a Naim cdx2 and xps2 powering a Pioneer A400. That's about £5k+ of cd player on a £230 classic amp. They fed Epos Es11s at around £500. Sounded great. Better once I switched to Naim amps and Zu speakers but not as dramatic s leap as you might think.
As per the previous poster. It's about knowing what sound you want and that's about doing demos.
Yes, the general guideline is speakers at TWICE the cost of the amp, but this is nothing more than a guideline. It gives you a starting point and sets a degree of proportion. You are free do deviate from this as you see fit. Some people prefer to dump more into the source or the amp and less into speakers.
Also as S.DJ points out the purpose for which you use the speakers, and how you use them matters, as well as the type of music you listen to. Are you interested in Hard Slamming bass, or are you interested in balance and clarity? Those are two very different systems. Though both could be done for the same budget.
Again, the 2x speakers + 1x amp is a very common and accepted guideline, and usually works out fine assuming you have a decent budget, and you do have a decent budget.
But there are more things to consider. Purpose, the style of music, how you play your music, are TV/Movies involved, how big is the room, etc...?