Hi,
Products without price are free products.
You can set the stock level for this product to 0.
After that there is many solutions.
You can activate the "contact button" on HikaShop.
System => Configuration => Display
"Display a contact button on the product page".
You can override the "product show_default" view (or "product show" view before HikaShop 1.5.6) in order to display a custom message if the stock level is empty.
Regards,
Jerome
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.