Today it would be over $300,000/year. I wonder what the price of oil was in 1938.
Somewhere between $1.00 and $1.14 a barrel. http://www.forbes.com/static_html/oil/2004/oil.shtml
Doing the math a little differently, if they got $30/month, that means a gross of $210 a month from the wells producing a little over 160 barrels a month, or a little under $1.31 a barrel. So, say roughly 1% of what today's cost was.