Free magnets?
Figure the cost of gas and hours to collect the HDs, a few sets of T-7/8/9 bits, hours & hours of removing them, cutting or breaking them, etc.
Then allow for the large percentage of broken, strange sized, too thin, or otherwise unusable magnets.
A lot of time and money, for a few mismatched hodge-podge magnets.
If you have the slightest difficulty obtaining HDs in large quantities containing similar magnets...
Buying magnets WILL be cheaper.
(I was waiting for Peter to pass out!)