A few things. It sounds like you want a manual transfer switch, which is fine.
#1 - What is the size of your service? 100A? 150A? 200A? That makes a difference on the transfer switch.
100A - $146.37
https://www.homedepot.com/p/GE-100-Amp-240-Volt-Non-Fused-Emergency-Power-Transfer-Switch-TC10323R/100171587
200A - $447.44
https://www.homedepot.com/p/GE-200-Amp-240-Volt-Non-Fused-Emergency-Power-Transfer-Switch-TC10324R/100150463
The electrician will need to provide some appropriately sized cables as well, depending on the size of your service.
Generally, to install the transfer switch, they will have to pull the meter. Depending on your local utility, they may have to do this. As a minimum, it will require reinstall of the meter and the utility tag. These may or may not be a charge. You may also have to submit a plan/drawing to the utility before they allow the meter to be pulled. And it may require a permit city/county permit in your area including inspection.
Finally, you will need a receptacle for the generator cable. for a 7kW generator, you will need a 50 Amp connection. $28.35
https://www.homedepot.com/p/Midwest-Electric-Products-50-Amp-Temporary-RV-Power-Outlet-U054P/100193650
And a generator cable. Price for a pre-made cable is around $185. If you make it yourself, around $100 for the plugs and 20 feet of 50A, 4 wire cable.
Cost of an electricians time to do this? About 1/2 day. Not sure what they cost where you are, but that would be about $500-$1000 here.
Does this help?