to House Repair Talk!
Cost is always a consideration in any roofing project. But most important is, are the $$$ you are spending, being spent wisely. Cutting corners effects how the new roof system will perform and how long it will last. My recommendation would be to mechanically attach the Liberty MA base sheet over a clean roof deck, follow up with the mid-ply and then cover with the cap sheet.
Why would you not use #30 roofing felt as a underlayment? Because it is not as strong and you run a very real possibility of wind lifting and the cap sheet will not seal to the organic base material like it will to the mid-ply. Staples will NOT hold as well as a nail. We use screws and plates to secure the base sheet in the field and around the perimeter. It's important that you use a 3X3" drip edge over the base sheets and then the cap sheet is embedded in roof cement around the outside edge. All lap seams should also be embedded in roof cement. Asphalt metal primmer should be sprayed on the drip edge prior to the roof cement.
Liberty is designed as a "system" to be installed, not a pick and choose what parts you want to incorporate into something you're trying to modify and shortcut. As a roofing contractor, what I see happen more often than not is premature roof failures by improper installation.
My advise, install a proper system, then you can expect the maxi mun life cycle out of it and get a better value for the $$$ you spend.