Surprisingly, there is a lack of data on what to budget for point of sale. The concept of a budget is frequently glossed over in manuals, books and ebooks - sometimes limited to only a token mention "create a business plan".
Ideally, an experienced retail consultant can be brought in as you are starting up your shop to really help you cover all bases, greatly increasing your chances of success in the long haul. If this isn't possible, we recommend speaking with successful shops who can help you determine what retail technology you may need to guaranty success in the long run, what features help them, what they wish they had, etc.
A solid rule of thumb is as follows: 2% of your yearly gross sales for the initial purchase of software, hardware, installation and training for a point of sale system. Retailers should ALSO be sure to budget 1% of their yearly sales towards maintenance, upgrades and the like.
Note that this is the GROSS profit, defined as - "total income from sales, salary, etc., BEFORE any deductions". Deductions would include inventory, payroll, rent and taxes.