If you are buying a new (say) TV, you can go from store to store and compare prices. If you know the exact model you want, of course you try to get the cheapest price. Whether you pay $2k $2100, $2200 etc, you are still buying the same product...right? So why pay more than you have to?
Choosing a carpet cleaning service (as with many trades) is very different!
You can find many in the yellow pages, local paper or in direct advertising that will tell you you can have a quality job done for around $15-20 per 'room' and maybe a package deal like 5 rooms for $99. BUT what is a 'room'

I SUGGEST THAT MOST OF THESE ADVERTISED PRICES ARE MISLEADING IF NOT DELIBERATELY DECEPTIVE.
Why? (A) Because they fail to qualify what standard of service you'll receive, and (B) because almost all advertised prices are ridiculously cheaper than they ought to be IF they are doing the job properly!

Without going into what makes the 'services' different, let me say that if you want it done properly, expect to pay at the very least $30 for an 'average' sized room of (<15 square metres). Depending on size, carpet type, degree of soiling, stains etc, paying around $45-50 for an average room is quite reasonable.
Getting a cheap job done is a false economy. It only supports the unprofessional element of the industry, and you take risks doing so!
Ash.