Costs
The strength of the British Pound (GBP) will make perhaps the biggest difference to foreign visitors, but even then Scotland is, by global standards, fairly expensive.
Budget
At the budget end (staying in hostels/camping, rarely eating out and traveling by public transport and visiting mostly free sights), you might be able to get away with about £30/person/day.
Standard
A more average budget for most visitors (staying at cheap hotels or B&Bs, eating out once a day and going to whatever sights they want), would be around double that, at £60/person/day.
High-end
Double that again and £120/person/day will buy you a pretty high-end experience, with smart hotels, meals at the full range of restaurants and your own car for the duration of your visit.
Luxury
But if you have are keen to explore luxury Scotland then costs can spiral beyond that.
Note that traveling solo will probably raise any of these per-person costs by at least a third for everyone except budget travellers.