OP, if the cost seems high or you like to make things, you can DIY. Google and you will find videos and webpages with instructions.
You can refashion spare brake fluid reservoir caps to make your own adapters.
Motive make plastic and metal adaptors. There is another company, Power Probe, that also makes the high quality metal adapters. If you want to buy the adapters then take a look at both companies as the designs differ.
You can also hook up air directly to an adapter, the idea being that you don't then need to clean and store a bottle. But the downside is you have to keep depressurising, remove adapter and refill and watch out for level more.
I prefer the hand pump Motive / Garden spray bottle with the quality metal adapters. The adapters designed for shop use will last forever for home use.
There is a Mercedes specialist, Kent Bsomething, on YouTube who is an excellent mechanic. He makes and sells the same product that Motive sell. Watch his videos for some ideas as his product has evolved to be easier to work with.