I am on my tablet now, so it’s a but harder to type the answer and add links etc. but you should be able to find one of my articles where I tested Canon FD 300/4 L against Minolta 300/2.8 and Canon EF 300/2.8 L IS. Canon FD did very well in that comparison and IMHO it is very good lens overall. I have two copies of it and both are very similar, so I won’t be afraid of sample variation issues.
I bought mine at about 500 USD each, which is not cheap but not bad for the premium quality product. I have no idea for how much they are going now, but I have seen some of the Canon FD gems such as 24-35 f/3.5 L going dirty cheap (200-250 USD), so maybe if you wait a bit, you might catch one of those 300/4Ls for a good price.