Hmm. Xstrap is an option on the pro bags. On the PAC website it lists all the options:
options check all you'd like to add to your order
Padded Waist Belt - - $27.00 ( ULTIMATE BAGS ONLY)
"X" Strap - $27.00(ONLY WORKS WITH PROFESSIONAL BAGS)
Extra Flap Extender - $27.00
Removable Pouch - $10.00(INCLUDED IN ULTIMATE BAGS)
Tube Holders - $11.00
Accessory Straps - $6.00 ( INCLUDED IN ULTIMATE BAGS)
Lock Loop - $22.00 ( NOT FOR STREET SMALL OR MINI)
Radio Holder - $21.00
Pump Holder - $10.00
Secret Pocket - $34.00 ( ULTIMATE BAGS ONLY)
Tom Binh modification to:
Ultimate / Ultimate O.S. - $23.00 - TOM BIHN MODIFICATION
Deluxe and Street bags - $8.00-TOM BIHN MODIFICATION
--------NEW OPTIONS !!--------
2 inch reflective strip on flap $11
mesh pocket across inside of bag $11 (like Ultimate Bags)
2 mesh pockets on either side of outside pocket $15 (Professional Bags only)

And the extra 7$ for a different interior color.

So for all the available [not included] options for the Ultimate bag without tax/shipping it comes to $505.