Value is too difficult to be accurate without seeing the gun. Condition which includes the amount of original finish and original configuration will determine value. Recommend taking it to a gun shop for appraisal.
There are many things to consider. Age, condition, scope... Say $3-600.
For that one, contact the Browning service department in Arnold, MO.
Try Browning.
Mounting an airsoft scope on an actual rifle would actually quite ruin it. Don't do it, because an airsoft scope is notdesigned to take the shock that an actual gun would give it.
There is no way of knowing without you providing the Model of Browning rifle that you own,its caliber,and a good description of your rifles overall condition.
Contact the maker.
You have a Grade I Browning takedown semiauto .22 rifle made in Japan in 1986. Browning offers a scope mount for that rifle, their item #12333. Aftermarket mounts are available from Leupold, Burris, etc.
$600-$700.
You need a good gunsmith
100-1000+ depending on specifics
there a a few things you can do you can mount it on the side it takes some getting used too but it works or you can get a special scope made for mounting farther forward there hard to find but they work great
The browning is typically your common sniper rifle with a scope added , but you could use a M240 , and put a scope on it , LIKE A BOSS