What type of construction? Are they doing the exact same prep steps, ie: floor leveling, although it need not be level just flat to within 1/8" over 10'-0".

Remodel in a beach/rental raised ranch home. Existing flooring is carpet. living room, hallway, 4 bedrooms, 980ft2. May require some subfloor prep/repair to ensure flatness as you mentioned. Stairs will remain carpet. Kitchen/dining is linoleum now but will be tiled soon.

‘May require some subfloor prep’. That is key

If these are VASTLY different costs, such as $3/sf vs. $8/sf, then the difference is the PREPARATION was mention in the $8/sf.

In my neck of the woods (Canada, expensive city like Vancouver, Toronto, Montreal, Quebec City, etc) the going rate JUST FOR LABOUR to install (the clicking of the planks) is $3/sf. Everything else is ADDED to that base number.

Carpet hides HORRIBLE subfloors. And sheet products (such as sheet vinyl) also hide HORRIBLE subfloors and the WORST surfaces for tiles. I would be prepared to pay $4/sf overall for subfloor prep. Cost of install of the tile and the vinyl will be extra. Cost of materials (tiles, vinyl, thinset, grout, etc) will be extra.

Does that help with what you are seeing? Can you list the two different prices for us? We'll tell you where things are missing and where you need to add more money (or not...depends on the SUBFLOOR).

IMO there is no way to answer this question but you need quotes that have all the same info to begin. IMO the floors all the same even in kitchen and DR would be the answer and for sure no carpet on stairs in any rental there are great ways to install vinyl plank on stairs too. Keep this simple and make sure you have at least 3 extra boxes of flooring for furure accidents.