I would check with the agent in the area that specialize in foreclosures ask them what would it take to get their business. You also will want to ask how long it takes to get paid. If you are desperate for money you may be waiting to get paid. Most Foreclosure's, the agents upfront the money and then have to submit billing to the bank, which in turn can take a while for them to get paid.
Ignore any advice to "charge $x [often around $1] per square foot." Bad, bad advice. Terrible advice. Your price depends on two primary factors: (1) your hourly rate, and (2) how long it'll take to do the job.
First element: Hourly rate. What is your time worth? $10 an hour? $20? $30? $40? Don't sell yourself short. Plumbers, electricians, auto mechanics, and others aren't afraid to charge what they think they're worth. You shouldn't be, either.
Second element: Number of hours required. First, you may work faster or slower than your competitors. That's OK. Second, some jobs will be easier. Some will be more difficult. You've got to factor in the length of time for each job.
Then you multiply hourly rate times number of hours. Let's say you figure it'll take you (or you and someone else, if you're going to be using some help) 20 hours to do the job. And you think you're worth $30 an hour. 20 times $30 is $600.
Then you add on your overhead. That's the time you spend on your business that isn't revenue-producing. It includes marketing, recordkeeping, driving around to provide estimates, etc. A good ballpark figure for overhead is 30%, but once you get rolling, you should adjust that figure up or (more likely) down. So, in our example, you boost $600 by 30%, and come up with $780. Also, factor in any special or unusual expenses--let's say the rental of a piece of equipment for a special task.
If you need to use a subcontractor to do some of the work (let's say flooring repair), find several companies that perform that service--wholesale, not retail. You don't want to go into your local retail flooring center and get a quote from them on repairing a 3' x 3' area on an oak floor. Find some suppliers up front. Generally understand what they'll charge, and make sure they can give you quick quotes when you need it. Let's say that a floor repair person quotes $150 for work. You'd take that figure and mark it up to cover your expenses and the services you'd be providing (a one-stop shop). A 30%-50% markup might be considered reasonable; it's up to you. So when you present your bid on cleaning and repairs, you present one number. You'll know what you're doing yourself and what you're contracting out for. The primary thing your customer wants to know is: What will it cost? And: When can it get done.
Then stick to your numbers. If someone else bids cheaper, let them. If you try to be the cheapest, there's always going to be some dummy who'll do it for less. Always. You want to be known for offering a good, dependable value. Or even (eventually) for offering an excellent high-quality service with a price to match. But start off offering excellent value at a fair price.
You might contact Building Service Contractors Association International. They're at http://www.bscai.org They have books and manuals and getting started in the janitorial/custodial business, how to do bidding and estimating, and how to expand into niche markets. (I used to work for them, and wrote a number of books on bidding and estimating.)
And here are a couple of other resources that might help: