hi, the difference can be very easily to understand in the following example: in Paris, if you want to buy some opium which , of course, you can not buy (买不到)it though you have enough money; while, let us go to another situation, see, one of your friend who is not rich for the time being, if you want him/her to buy one airplane as a birthday gift to you, he/she , of course, can not afford to buy(买不起) it. hehehe, is it clear? You are welcome to ask any question on Chinese langauge.
Derek