Unless the 20K home has been foreclosed and is bank owned or is being sold by an injunction from the town for the back taxes, I would avoid that nightmare at all costs. There is another reason why a house is being sold for 20K and the answer is that it needs a lot of work, work that you probably cannot "see", like bad plumbing or bad septic problems, termite damage, maybe the structure is not sound, the windows are old as the hills or the electrical panel needs complete upgrading. All things that cost thousands of dollars to repair or replace. Can you honestly say that you see zero physical difference between the 20k house and the 75k house? There has GOT to be a reason. FIND OUT.
If you are financing the house, the bank will REQUIRE an inspection. This inspection will reveal all of the issues the house has. But, it is likely a house of that price range is going to be sold "as is" which means the inspection is for your knowledge, not so you can negotiate the price or demand any repairs....in which case, no bank will lend you money for that house.
Due diligence is needed here. Call the real estate agent and ask if the house has been inspected or if it had any pending offers that fell through and if so, ask why. Go to Zillow.com and look at the listing there to see the price history and find other information about the house. Even googling the address can net you some information about the property. Who knows, perhaps there was a triple murder there!! Hey, anything is possible.
KNOW THIS: any 20K house that is comparable to a 75K house MUST have some issues. You just need to know what they are. You can also find out information at the local municipality (town hall) to see if the place has been condemned or when the last roof was installed (that is what permits are good for, you get to see when work was last done...if a permit was pulled, that is).
Find out all you can about that 20K house before you even consider buying it.
Newsflash: Every house you buy will need regular maintenance. They all do. I built my home in 1986 and it is time to begin repairing and replacing things even though I kept up the maintenance. The windows, with new window technology, for one, is first on my list to replace.
If you cannot throw away a few hundred for an inspection, at least get some construction friends or people who know the trades to come and look at the place for an unofficial inspection before you even make a bid.