They are coming.  
 
Amazing that three words can scare the living daylights out of a person.  Company, family, in-laws, The Kid, That Guy - and guess what, they need a bed to sleep in while they're at your house.  And no, the wife says they won't be sleeping outside - Plan A is out the window. 
 
Shopping for a mattress shouldn't be a headache, especially when shopping for a spare one.  There are a couple of ways to do it that will make sure that you get the best bang for your buck.  
 
Time - while looking into the quality and comfort of the bed so that your guests are comfortable, time should be just as important of a factor as comfort.  Just how long are they planning on staying, and how much is that bed going to be used? A couple of days to a couple of months, twice a year to four months out of the year will play a big part on whether you'll be taking that low end mattress set.  While buying a cheaper mattress might save you some money in the short term, having to replace it in a year because of heavy use doesn't seem very practical, does it?
 
Protection - always buy mattress protectors.  There is no reason not to have them.  Mattress covers serve two purposes in this situation.  One: having one on the bed means that the mattress will be protected against anything that may come off the people sleeping in it.  Sweat and other fluids can easily be cleaned off a protector, once inside the mattress, they aren't coming out.  This also helps if the mattress is something that you're only going to have in the short term - a clean mattress has a better chance of being taken by someone who needs it once its served its purpose at your house. 
 
Another option one can consider - use your own mattress for company.  Buy a brand new one for yourself.  Not only will you be comfortable, your guests will literally have the second best bed in the house.  You should know its comfortable, you were sleeping on in just the other day.