On the Customer Details you have 2 settings that affect this:
Customer Shop Status - View Shop and Prices
Customer must be approved to shop
0= Not required
1= Must login to browse
2= May browse but no prices unless logged in
3= Showroom Only

It is recommended that Option 2 be used for the purposes of Spiders if you wish customers to login to see prices.
Customer Approval Status - Authorization Pending
Customer must be Authorized to shop
0= Not required
1= Must be Authorized to Browse
2= May browse but no prices unless Authorized
3= Customer May Browse and May see Prices but Must be Authorized to Buy

It is recommended that Option 2 or 3 be used for the purposes of Spiders
NOTE: when testing be sure to start with a fresh browser session as the settings are managed based on these settings when the customer hits the site the first time ...

I am not sure what version Zen Cart you have as we do not have a v1.5 ...