A lot depends on what you are actually GETTING for your money, what sort of gigs you are doing, and the upstream audio in the chain before it (Mixer, any EQ Products etc), since your audio is only as strong as its weakest link, and it would be daft to connect a £2000 P.A Rig to a £39 Mixer with no EQ!

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If budget is what you are looking for, and you are just starting out and money is tight then there are reasonably respectable pairs of small 100 - 200W RMS Cabs in the £150 - £250 / pair price bracket, but don't plug them into a 1000W Amp

and don't expect them to cope with anything more than a small hall or pub with more than 100ish people .
At the end of the day you'll get what you pay for.....but only to SOME extent, and it can be overkill to spend £2000 on a system if you are just starting out or only do a weekly gig at the local boozer, or the Kids Disco at the Youth Club.
There is more involved to building a loudspeaker than just throwing a 12" driver and Piezo into a cab and hoping for the best

, but often you can end up paying just for the badge on the front of the speaker.
Many respected speaker manufacturers use the same Drivers, crossovers and horns as their competitors, and one of the biggest OEM speaker manufacturers is Eminence - who used to have the lions share of selling drivers to the big name Loudspeaker Market. So ultimately you may buy a speaker for £400 and then find a similar cab using exactly the same Driver and components for £290!, what you pay all depends on the manufacturer, their mark up, how "famous" they are, and the overheads of their distributors, just like anything else in the consumer market -Big name Trainers for example!.
There is a glut of cheaper loudspeakers coming onto the market from the Far East. Some are reasonably okay, and quite good quality for the money. Others are lemons, using the cheapest components possible often Automotive or uprated hi-fi drivers and not made for anything other than bedroom use!.
Beware of dealers exaggerating the Rating of the speakers, it has been stated by me 1000 of times, but I will say it again....ALWAYS GET THE CONTINUOUS RMS RATING of a loudspeaker before you buy!. Dealers may try and push the power handling of cheaper products using Peak, PMPO or Maximum figures!. For example a 600W maximum rated speaker, may only comfortably handle 200W RMS of power for very long periods!

. Connecting a 400W Peak rated Speaker to a 1000W RMS Amplifier will prove my point very quickly

. If in doubt look at the method of connection into the speaker. If it uses a combination of Jack Socket and Banana Type Plugs / screw terminals then the power handling is unlikely to be anything above 200W RMS. If it uses Jacks only then no more than 300W RMS. Most speakers nowadays use "Speakon" Type sockets, and its almost certain that any professional cab rated above 300W RMS will only use XLR or Speakon type connectors. If the speaker that you are looking at is marketed as being 700W and it has banana plugs on the back of it, then walk away very quickly!.
Final Tip:- When choosing cheaper speakers, always look out for the ones using compression drivers or horn driven tweeters. They sound far nicer and better quality than the piezo type. Also look at build quality, and trust your instincts!.