Q: How hard is it to seal the drafts in a basement?

A: When I air sealed my basement there were many small holes and a couple of large ones. One was about one foot square and was just above the sill plate (where the concrete meets the wood framing). You could not see through it, but it obviously connected to the outside probably through hollow walls to an overhang. It was too big to seal with a can of spray foam (I tried) so I stuffed the hole with fiberglass and then coated the fiberglass with a layer of spray foam. The foam stuck nicely to the fiberglass and then also to the wood framing around the hole. One can of foam did it. The other big hole was a hole drilled through the concrete wall, probably for an electrical run but there was nothing in the hole. It was a great way for mice to get in. You could see daylight though the hole. This I just filled with spray foam. This strategy might work well for you. Most of the drafts in a basement are not caused by wind but instead by the stack effect which is where warm air rises sucking cold air in through the holes in the basement. Now we have cold weather this makes it easy to find the holes – you can just feel the cold air rushing in. Seal the biggest ones first and it will increase the flow rate of air through the smaller ones making them easier to detect. You can also detect them with an infrared camera. Libraries often have these available for loan. A cheap alternative is an infrared thermometer. Mine, called the TLD100 is available on Amazon for $30 https://smile.amazon.com/BLACK-DECKER-TLD100-Thermal-Detector/dp/B0044R87BE/ref=sr_1_5?crid=11SQIH456HZHU&dchild=1&keywords=tld100+thermal+leak+detector&qid=1605307783&sprefix=tld100%2Caps%2C237&sr=8-5
Once you have done this you can start a second career as an air sealer. One guy I know gets paid $75 an hour!