You might be better off with a custom cut-&-sewn set. Probably the only way to get an exact fit for such a modded floor.
New underlay, the custom made carpets, including a new heel pad, installed, would run you about $400.00/$450.00 in my shop. That includes the materials. Take me around 4-5 hours to do. Shop rate is $50.00/hr. The materials are in the $200.00 range for in-stock colors.
If something like German Square Weave carpet was spec'd, better sit down, 'cause that material runs into the $150.00+/yd. range.
You'll need about 4.5-5 yards of carpet, & 4 yards of underlay, just for the floor. Add another 2.5 yards of carpet to do the back wall, plus the additional time involved with that.
But as these are made specifically for the vehicle, I'd need it at the shop, so, I just couldn't do them for you! Much as I'd like to.
Check with your local upholstery guys. At least now you have an idea of what to expect when getting this done.