We visited on Tuesday 5/4/21 and it was free.
Click on the Fees and Passes section in this website for current info. https://www.nps.gov/wupa/planyourvisit/basicinfo.htm You'll see the fee includes entry to Subset Crater Volcano National Monument. If you have a America the Beautiful National Parks Pass or another means of free entry, it may be free. If you are going to other nearby places like Grand Canyon, Horseshoe Bend, Walnut Canyon, the pass can be well worth it. It is also valid and National Parks across the country. https://www.nps.gov/findapark/index.htm
Private vehicle cost is $25.00.
We visited the monument on Thursday, May 6 and it was free. Visitor centers were open with limited access. Masks required. Some places were closed due to Covid.
Its free
Thanks! Your answer is awaiting moderation.