Symbolic Logic, D. Jacquette, Wadsworth, 2001, Chpt. 8, IV(15), p. 435

The argument:

"Something is costly and something is free. Something is prohibitively expensive only if nothing is free. As a result, nothing is prohibitively expensive."

The proof:

1. (∃x)Cx • (∃x)Fx
2. (∃x)(Px• Cx) ⊃¬ (∃x)Fx
3. ∴¬ (∃x)(Px• Cx)
4. (∃x)Fx ......... 1 Simp.
5. ¬ (∃x)(Px• Cx) ......... 4,2 MT