Its a bit of a 'how long is a piece of string' question or answer
You can set your bids manually say 1.20 per click which would get you onto page 1 for someone typing in 'marble floor cleaning' but you could only be in position 6 on the sponsored links which means you are on the right hand side of the page. if you drop down lower than position 3 enquiries drop off. So to stay in the top 3 spots you need to bid more per click - so you can see where the money pit comment came in. You can cap your budget to control to stop runaway spending but that means your disappears when your budget is spent so there's obviously no chance of enquiry after the budgets gone.
Just to make this more confusing/annoying you can sometimes get a higher position by getting a better quality score from google. If your adword advert appeared as 'marble floor cleaning' and underneath it said something like 'marble floor cleaning service Harrogate' and the advert pointed to a landing page on your website which was titled 'marble floor cleaning' and on that page all it talks about was marble floor cleaning and the website itself was called
www.marblefloorcleaning.co.uk that advert will get a better quality score than a general cleaning site that just talks generally about hard floor cleaning. So you could pay less per click that a competitor but appear higher up the page.
You can make your budget go further by tailoring the key words to stop useless clicks from people seeing your add who are not really interested - they type in 'marble floor cleaning products' and your add pops up as google thinks its relevant, you get clicked on and pay £1.50 for nothing you are not selling a product you are selling a service. Put the word 'product' in as a negative keyword and hopefully you will not come up.
I have been trialling adwords and its better than most forms of general ads but it is hard to get it right I certainly have not mastered it. I guess if you ever really nail it you could just sell yourself as an Adword consultant. There seems to be quite a few web designers who offer adword campaign management I would bet money a lot do not know that much and in particular do not understand industry specific marketing. It is still trial and error for me and has not made me rich the enquiries are not coming thick and fast but I do tend to convert most I get. Its getting me off carpet & upholstery cleaning and I could do with a better website so its small steps.