Cost to get from Lauterbrunnen to Schilthorn

That plan is fine - plenty of time. Lauterbrunnen to Schilthorn costs about $110 roundtrip, if you're not using a pass or half-fare card. See also https://schilthorn.ch/.

You could do both in a morning. I'd suggest Lauterbrunnen to Grutschalp (cable car) then train from Grutschalp to Murren. After checking to see if the summit is clear (there's a monitor in the Murren lobby) go up to the Schilthorn and down and then take the cable car from Murren down to Stechelberg and from there it is a walk or short bus to Trummelbach Falls.

More info if you click on my name and follow the link there and check under "tips".

Thank you both. This group is so great for information

And the ticket you purchase will let you stop off along the way in Gimmelwald and then stop off in Murren and also Birg. Have a look around and then continue on your journey with the same ticket.

Prices are here:
https://schilthorn.ch/cmsfiles/tariffs_schilthorn_2019_english.pdf

You may want to price out the Swiss Travel Pass. The Schilthorn has been included in the pass, not sure of the status right now.

Yes, it’s included.

So, I'm doing a driving tour and therefore, no trains. Does it cost 105 SF for a round trip up to the top? Is this worth it if I have limited time in the area?

Does it cost 105 SF for a round trip up to the top?

marshalbill ,
Yes, if you don't have any discounts. See pricelist (as posted previously): https://schilthorn.ch/cmsfiles/tariffs_schilthorn_2019_english.pdf
Stechelberg is the valley station, past Lauterbrunnen. It is also the end of the road, where you have to park.
You will have to pay for parking, rates here: http://www.stechelberg.ch/service/anreise/parkplatz-schilthornbahn.html

Is this worth it if I have limited time in the area?

Yes, depending on personal preferences and if you have anwhere you would rather go.
Only go up if the weather is clear, not if the summit is in clouds.