Bring in an independent inspector (not one thru a realtor), which may lead to consulting with a structural engineer.

Maybe I'm wrong, but I think you'll go over 30K to fix that type of problem. I'd keep looking.....................

Having serious foundation problems in our old home, I would strongly urge you to do what has just been suggested....get an excellent structural engineer to study everything and don't use one suggested by a realtor. Good luck!

Thank you for the responses

What if it turns out the main beam has been replaced and this is just a leftover cosmetic problem? If it bothered me enough is there a way to level the floors? I really like the house otherwise

"...is there a way to level the floors?"

Yes, but it still depends on exactly why they are out of level ior if they are actually out of flat.
If the joists have sagged from age/size (a dip between supports) they are out of flat and removing the floor, adding or replacing joists, and the putting the floor back down may be required. If the area under the floor is accessible (not finished) you sometimes can get away with sistering new wood that is flat, but the old wood has often taken a set that may not be easy to remove.
If the floor is out of level since one end of the joists are not level with the other, major structural work may be needed, or have been performed in an inadequate manner.
If a main beam was already replaced, it may be that the work was poorly done (not aligned with the outer walls) or that the outer walls have continued to settle resulting in the beam being high.
If it is a small amount you might be able to lift the outer walls, but this is a large task.
Expect to spend a lot of $ on the repair, then more on the wall cracks that these types of repairs often (nearly always) lead to.

You don't say how old the houses are that you are looking at and not knowing the soil structure or climates, or whether the houses are on a slab, foundation or frost wall, it is hard to tell what could be going on.

All houses will settle with age, that is the foundation and lally columns and footings will sink and move depending on the size of the house and the type of soil, type of footings, etc. A house over a hundred years old will often require some jacking, leveling and shimming of main beams and/or lally columns eventually. A field stone foundation not put together well, or on soft soil may be prone to move as well.

In newer houses, especially those built on a slab as is common in some parts of the country, the slab can move and crack, causing irregularities to occur.

You mention one house with an addition that was lower than the main house. This is a common problem if the planning for the new addition adjoining the house is not done correctly; not finding the proper elevation to the existing floor joists, or a structural failure due to poor construction.

Expect to pay an engineer anywhere from $500 - $1500 to get an assessment. Engineers have to be board certified and licensed, get the opinion of a couple before paying any one individual for a set of plans.

Thanks for all the good advice

You don't say how old the houses are that you are looking at and not knowing the soil structure or climates, or whether the houses are on a slab, foundation or frost wall, it is hard to tell what could be going on

To answer some of the questions. It was built in 1950 is a shingled Colonial. The problem is mostly on the second floor. Sort of smack in the middle of the house where it seems higher in certain places. The house has a full underground basement and is about 3500 sq feet, probably 30 feet wide and 50 feet long. It is in Long Island, NY, near a pond and not far from a channel leading to the Bay which leads to the Ocean. I believe the walls are plaster if that matters

Good point about any jacking then having an effect on the walls and ceilings

The main beam is metal which is unusual if it is original, usually that means it was replaced at some point. There is a much larger house from the 20s next door, also for sale which has exactly the same problem. I did not go into the basement of that one so no idea what it looks like

"The problem is mostly on the second floor. Sort of smack in the middle of the house where it seems higher in certain places. "

Is the "main beam" in the basement?
What joists and walls are present to hold up the second floor?
There should be a weight bearing wall (may be in sections) directly over the basement beam to provide mid-span support for the second floor.
Has someone removed the weight bearing walls?

Find a structural engineer with house experience and pay them if you are really interested.
Without the ability to punch some holes to see inside ceilings and walls it may still be only an educated guess though.