Hi!
We have just released a new module for X-Cart 5 -
Hide Prices.
http://www.x-cart.com/sites/default/files/addons/image/Hide_Prices_184px.png
This module gives you the ability to disable checkout and hide prices for users that are not signed in.
You can change the "sign in to buy" message (that appears instead of the "Add to cart" button) in the "translations" section of the back end.
Feel free to ask me should you have any questions or suggestions for the module.
Thanks!