There are several services that can estimate the "value" of your Steam account by calculating the current price of the games included in it. But you can see exactly how much money you've spent on Steam games since you signed up for the service through Steam's own tools.
This refers specifically to external money you entered into Steam, meaning that gift games, game keys purchased in other stores and activated in Steam, money earned from selling cards and inventory items, etc. are not counted.
To do this, in the Steam application, select the Help menu item and follow the following path : Help > Steam Support > My Account > Data associated with your Steam account > External funds used. Additional authorization in SteamGuard may be required.
For the English version of Steam, the path is as follows – Help > Steam support > My account > Data related to your Steam account > External funds used.
In the previous menu "Data associated with your Steam account" you can find other interesting data: purchase history, cart history, subscriptions, favorites, recent support requests, gift history, exchange offers, family settings, workshop payments and much more.
Thanks to Mykhailo Zinchenko, who reminded us of this opportunity.