Simple really cost, performance, support and client side features:
Checkpoint cost is:
- Cost of hardware platform + support of platform
- Cost of Checkpoint firewall softwware + support of platform
- Cost of client side software (if using SecureClient) + support of platform
Sample UK pounds cost would be:
- Nokia IP330 £6k + £800/yr
- Combined FW+Mgmt £5k + £400/yr
- Cost per user £50
Total = £11k + (£50 * No. of users using SecureClient) + Support
Cisco cost is:
- Cost of hardware+software + support of platform
Sample UK pounds cost would be:
- Cisco 515UR = £7k + £800/yr
- Client software (include personal firewalling) = £0 * No. of users
Total = £7k + £800/yr
I think the mathematics is very simple
On the other features, PIX is faster CPU for CPU as it has no OS overheads, same if not more features than Checkpoint. Checkpoint relies on firewall admins having multiple skills sets (OS vendor + Firewall vendor) therefore cost more

. The one selling point for Checkpoint in the past was the GUI, but with the PIX PDM (And Cisco Secure Policy Manager) this is no longer the case.
The one feature that Checkpoint maybe still excels at is logging in the GUI, however what it logs is of very limited use for Intrusion analysis, whereas the PIX does log this data the syslog format isn't very human friendly.
I'm certified in both, and use both daily, but much prefer the PIX for all of the above reasons.