Cheapest are around 300k but you can't store anything in them and they are just a spawn point.
Getting a 500k one will give you space for 1 box and they are pretty small. The 1m houses give you 2 - 3 spaces and have more than one room generally.
1.5 - 2 mill houses are generally space for 4 boxes and have multiple floors. They are the biggest but also the biggest target for robbery.
I would recommend having say 2 500k ones with 1 box in each (in the middle of nowhere) as they will be less likely to get robbed and give you more spawn locations to get picked up from etc.
I recommend you leave yourself at least 1 space for a garage (you can get 3 houses maximum) because if you manage to find an unowned one in a good place they are quite nice to have.