Look the link; some good explanation.
Take a summary :
10% Planning and pre-development tasks
· 55% E-commerce system Configuration and Database development
· 25% Design
· 10% Project Management
How Much Does an eCommerce Website Cost in 2018?
DOMAIN NAME : Cost: $5 to $15 per year
WEBSITE HOSTING : Cost: $75 to $200 a year (depending on traffic and hosting features)
CONTENT MANAGEMENT SYSTEM (CMS) : Cost: $0 to $15,000+ per year (depending on the platform chosen)
ECOMMERCE FUNCTIONALITY : Cost: $500 to $5,000 (initial investment)
WEBSITE DESIGN OR THEME CREATION : Cost: $250 to $20,000+ (initial investment)
COPYWRITING : Cost: $50 to $500 per page or article
SEO : Cost: $300 to $1,500 per month
MULTIMEDIA DESIGN : Cost: $250 to $10,000+ (depending on level of interactivity required)
DATABASE INTEGRATION : Cost: $5,000 to $25,000 per integration
ONGOING WEBSITE MAINTENANCE : Cost: $500 to $1250 a year (depending on number of updates required)
That's the basic, after there some application you can use inside the market place free or not free should be increase the cost. no ?