Penn actually requires both all SATs and all ACTs but you can send whatever subject tests you want to send. Subject tests are no longer rrequired but two are recommended. You should not have an additonal financial issue sending SATs if you send sebject tests. College Board will send all your SATs and subject test scores to a college in one order for the same price, and thus it does not cost more to send SATs with subject tests.
If you have a true financial issue in sending any scores, both SAT and ACT have a process which allows you to send a certain number of free scores without charge, in additon to the four free ones you get if you identify colleges in your test registration to receive scores upon release, but you have to apply when registering for a test and your family needs to meet certain low income requirements.
As to what Penn does if you send both ACT and SAT, it professes like others that it uses that test, SAT or ACT, which is more favoarable to you for admissions, and does not hold the other one agaisnt you.