The Cost Between Saas and Open Source eCommerce Platform

*** Opensource ***
Developer or consultant fees if it needs
System Integration to other system like ERP
Addon Apps or additional modules (usually, it charges the initial setup)
Hosting Service Charges – It depends on the provider and how much bandwidth and speed you need)
Support service fees (It depends on the service level and your subscription plan)
Free of charge at eCommerce framework (usually it is free)

*** SaaS ***
Developer or consultant fees if it needs
System Integration to other system like ERP
Addon Apps or additional modules (usually, it charges per month)
Support service fees (It depends on the service level and your subscription plan)
Monthly subscription fee at eCommerce framework (It depends on the platform and service level)

So, all in all it really depends on your budget and planning.
If you want to run your business for a long term, the cost of opensource is definitely cheaper than SaaS.
If you want to have a test on your business, the startup cost is cheaper with SasS.