A lot of times you hear that people that are religious ask for money because they are learning Torah. This seems odd. One reason is the source the Gra brings to the mishna in Avot "not to make a shovel of Torah to di with"--i.e to gain profit from. The source the Gra brings is from the prohibition of ""meila'. That is using something holy to make money from. For example someone says this ox I am going to bring as a fire offering. Well the ox then is forbidden in use. So if someone uses it--lets say they plow with it. They transgress this prohibition. This applies in three categories. Presents to the Temple. Animals dedicated as sacrifices. Oaths. [As the law is like R.Meir who says יש מעילה בקונמות.
To use something holy to gain personal benefit or profit is a sin
To use Torah to make money is the same as if one took the ox and plowed with it and say I am not really plowing. I am simply walking with this ox that just happens to have a plow attacked to its back--a dn I am just taking a walk in my field.
[TheRambam brings this same point in Pirkei Avot chapter 4.]
