Hi Miriam,
I think budget issues are always a consideration. I'm with you: if the owner doesn't get that this is a long term effort to increase their business, then I tell them I can't help them.
I tell them that my job is to increase their business and that I need a commitment of time and money to do so. I generally try to get them to commit for a one year period and a budget large enough to actually accomplish something.
These kinds of customers are harder to find, but, once I do, because I have the time and money to get a result, they tend to become more or less permanent customers.
My biggest challenge is explaining what I'm going to do and how this will result in more business.