While mold is indeed ubiquitous in the environment, what grows in houses tends to be well beyond "normal" limits, and can very definitely be extremely hazardous to health, particularly if it is one of a number of varieties that are considered toxic. What makes them hazardous is not only the types involved, but the concentrations, and usual lack of ventilation around where they are growing. Even with the "normal", nontoxic types, many people are sensitive to molds, particularly in large amounts, and can become quite ill from it. By the time you are able to see it on the walls, you can bet there's a great deal more behind the scenes (yes, hidden behind and in the drywall, as well as possibly permeating the studs and floor joists), and if you can smell it but can't see it, you're likely in for a major job to try to locate it and remediate it. The big problem is that once you find a mold problem, you not only have to get rid of that but you've also got to make sure that the reason it got there in the first place is eliminated. If there's a hidden leak somewhere - like the roof - that could be a major job to fix. If you can stop the cause, chances are good you can stop the mold. I'm afraid that it's impossible to answer the question of how much it will cost to fix any given mold problem without a complete evaluation of the extent of the problem. This is a well-known hazard, and I had the misfortune to have to learn more about it than I ever wanted to know because of finding it in my own home a few years ago. Proper environmental testing will actually cost a great deal more than $50, and I absolutely recommend hiring an environmental hygienist or engineer to do it if you like the house and want to still consider it.