I am sorry, but that is a question most people learn to answer in school.
- You are quote a price X for a piece of land.
- The land is somehow defined with a size.
Now you calculate the number of square foot from the size, then you divide the total price by the number of square foot. Done. Land is often NOT quoted in square foot as the unit is WAY too small. Depending on country I have seen Are (100 square meter), Acre or Hectar as unit, both on leasing as well as purchasing.
I would find it amazingly funny if you would be offered land without the proper defining terms (i.e. a map, exact definition what you rent and that basically tells you the size). I know in some countries people are not doing that (rent in the UK - number of rooms, but no sizes given unless unusually large), but this is really an indication of something not kosher (rooms are so small that a bedroom fits a bed, not anything else). But seriously, if you get a piece of land, both location and size are standard, and if you are unable to calcualte price per whatever land unit from the price and the size, you do have FAR larger problems than a missing price per square foot.
Does this look unprofessionally from leasing agent perspective to behave this way?
Yes and not. Land may have limitations what you can do and he may expect you to have those things ready.
and the advise for me is to always prepare my answers prior contacting them?
You should ALWAYS be prepared for this type of question, particularly if you are leasing land or non-living property. This is rental 101. There may be limitations (both legal or by the owner) and you answer or get lost.