The questions about the price is out of topic here because they are impossible to answer "universally", for the answer to be useful to more than one person, especially when no details about the existing setup are given. So let me instead outline a structure of the costs.
A new 1×11 groupset (cassette, chain, derailleur, shifter, front chainring, possibly front crankset, possibly new rear wheel) costs anywhere from 200 $/€/£ to 2000 and more, depending on what level of performance you want. An existing rear wheel may be sometimes preserved (again, we do not know your setup), but if you go for 1×12, you'll definitely need a new rear hub (essentially, a new wheel).
Smaller stuff, such as new shifting cables/housing, are worth as well, costing you maybe additional 10—30 $/€/£.
If you buy components online, add another 10—100 $/€/£ for postal services. If you buy at a bike shop, add implicit higher margin they have compared to online shops.
Then, there are installation costs. They can vary wildly: from zero if you are ready to learn the skills yourself, can loan specialty tools and have a lot of time to learn required skills, or have a friend that is willing to help you for free, to what price is for 1 or 2 hours of work of professional bicycle mechanic in your area.
Note that for these money, it is often possible to buy a complete new bicycle with entry to medium level of a 1×11 drivetrain. Selling your existing bike would even offset the cost a bit (I doubt that by much).
There are also cheaper wide range 1×10 groupsets (and even 1×9 and 1×8 by Microshift), which maybe are a better choice for upgrading an older bicycle.