While a higher price certainly does not guarantee a better "product" (education, experience, etc), I'd be wary of targeting explicitly lower cost preps.
The combination of cost of living/lower birth rates/possible VAT on fees means not as many students are enrolling, and the less "desirable" schools are having to compete for students. If they don't have the reputation or facilities, one way they may choose to compete is on price.
At the end of the day, though, most private school "consumers" are relatively price insensitive (especially in central London) and will choose wherever they think is best for their kids -- and that's going to leave some lower tier preps struggling and even closing. There have already been a couple of closures and mergers around our part of SW London.
I agree with other posters here -- rather than looking for a unicorn "good value" prep, get them into a good local state school, supplement with tutors, extracurriculars, etc, and save up for secondary. School fee rises over the past 20 years have far outstripped inflation and pay rises, and they show no sign of slowing down, so think long and hard before making this commitment.
I say all this as a fellow mum about to send her first child off to a prep this year, and thinking about the massive financial commitment ahead!