I don't have direct knowledge because I've not used such services myself, but I have tried to help people recover after using them.
$300 divided by 40 = $7.50 per link.
I don't live in the States or deal in dollars, but here in the UK £4.65 might get you a nice sandwich.
What value per hour would you place on an SEO's work? And from that, how much time do you think would go into a $7.50 link?
Factory SEO like that does still seem to work sometimes (at least sites that have done it are still ranking) but I can't work out if the people that do it are stupid and lucky or really clever. One day I might try testing it myself to see which it is :)