What is the easiest and most inexpensive way to insulate floor joists?

It's not that cold air is getting in through the floor, but rather that upstairs heat is being transferred down through the floor to the cooler basement. Insulating the floor joists above the basement is the best long-term solution to heat transference. An R-13 fiberglass roll insulation (15" x 32') will cost you roughly $20-$25 per roll. Rigid foam-core insulation only has an R-3 to R-5 rating and will cost $13-$20, but covers a 4'x8' area, so less effective but less costly. The rigid foam-core will go up like a ceiling in the basement.

You can instead add an area rug or wall to wall carpeting (both should have padding for comfort and carpet longevity) to insulate against the cold floor. Check this link for more complete information on fiber types: http://www.carpetbuyershandbook.com/carpet-basics/construction-fibers/pile-fibers.php

Easiest is to have someone else do it. It is not the most inexpensive way so you have to choose.

https://www.todayshomeowner.com/video/how-to-insulate-under-floors-in-a-basement-or-crawlspace/

The loose fibreglass insulation is okay to use but I would cover it with at least a 4 mil or 6 mil plastic stapled to the floor joists.

I recently insulated the floor joists in my living room and kitchen with fiberglass batts from the crawl space. Two days on my back in a three foot crawl space putting up fiberglass insulation. The floors are much warmer and no cold air from the crawl space. If the basement ceiling is open just get fiberglass insulation in rolls and fill the floor joists. Get insulation the same thickness as the depth of the joists.