Cost varies depending on the payment processor and the country. Stripe fees are 2.9% while your own merchant account will be lower than Stripe but based on risk assessment by the bank.
The application process via Stripe will allow you to immediately start processing payments while the bank application can take up to 24 hours for approval. Both methods will require you having the card reader to start.
Square, Revel, and Shopkeep are good products but they are only electronic cash registers. Costbucket POS, in contrast, include a fully functional accounting software with inventory management.
The total cost depends on the package chosen and the payment processor used. For example, the POS & Accounting package for 3 users cost $45 per month plus your monthly card processing fees per successful card transaction.