I've done over 1000 end of tenancy cleans, whilst they can be hard work, they can also provide your business with regular income (particularly if you work for letting agencies). People mean different things when they ask for an end of tenancy or professional standard clean, there is not a universally agreed definitive standard. I advise anyone wanting to offer this service to specify in writing what is included and highlight the main exclusions or limitations. I offer three levels of service, Ultra, Standard and Value, most people go for the standard service (which satisfies the vast majority of landlords and agencies). For my standard service I would charge around £400 for an average 4 bedroom house (1200 -1500 sq ft), including carpets, windows and oven. I would expect it to take around 20 man hours to complete (to a high standard) by experienced cleaners. The Ultra service might be closer to £1000 and include curtains, upholstery and specialist hard floor care, but very few people will pay this much, so I do not do very many.
The money is less than I would charge for stand alone cleaning services. However you can still make a good margin by employing cleaners to do the low value cleaning, whilst you or your specialist staff complete the more lucrative parts like carpets, windows and ovens.
Also you should also expect to have to go back to a proportion of your jobs. Some of your clients and landlords will be very fussy and ask you to return to rectify even very minor oversights or deficiencies. For my business, we generally return to about 3% of EOT jobs to address issues, however for a beginner the rate might be a lot higher to begin with.