... If you get Ultimate, does that include all the core 5e material?
A Fantasy Grounds license costs $39. If everyone in your group has this license, then no one needs an "Ultimate" license. The ultimate license for $149 allows unlicensed free installs of Fantasy Grounds to connect to the ultimate license holder's hosted game. The licenses are also available as subscriptions, $4 for the regular license and $10 for the ultimate.
The basic installation of Fantasy Grounds comes with both the 5e basic rules and the SRD, in separate modules that you can use independently or together. Those rule sets are free. No license for Fantasy Grounds comes with the PH, MM, DMG, or SCAG - those are separate purchases. The three core books are each $50 and the SCAG is $40. There are a couple of free adventures for 5e, others range in price from $5 through $20, and the official adventure paths like Out of the Abyss are $35.
Only one person (the DM) needs to have the books, other users connecting to the hosted game can use any of the books that the DM sets to "public."
What that means for groups using Fantasy Grounds is that if you want to try it out cheaply, the DM can spend $10 per month for the ultimate license, use the SRD material, and input anything else the group needs (including homebrew material) manually (copy/paste works.) This is all you need to do for a full 5e virtual tabletop experience. Honest. You can run a complete and awesome campaign this way, for just $10 a month.
On the other end of the spectrum, you can have each member of the group get a license (so no one needs the ultimate) and everyone can get their own Player's Handbook module (so they can create characters and explore level-up options offline, etc.) while the DM gets the DMG and MM modules, as well as perhaps the SCAG and a WotC adventure path. You certainly can spend a lot of money on Fantasy Grounds and related products, and honestly if you do, you get your money's worth out of it (especially as a DM.) Time is money, and you save
so freakin' much prep time using the official licensed products. You get full drag-and-drop functionality with those modules, the DMG comes with fun toys like the "item forge" (and rumor has it the "npc forge" is being worked on.) A published adventure module will have everything ready to go, with the maps marked-up for the GM and the NPCs bundled in encounters ready to drop into the combat tracker, and treasure parcels ready to drop onto the party sheet to be assigned to characters.
If you're unsure, my advice would be this: install the software (free) and mess around with it some. Look at the tutorials, wiki, videos, etc. to get a basic feel for the functionality, then find a game to join for one or more sessions. There is an active Fantasy Grounds forum community, and from time to time SmiteWorks will arrange a virtual convention, FGCon, where people can play using the free software and experience a game run through the VTT. If you and your group want to jump right in, you can actually have a pretty fun first session on the VTT figuring out the software and getting everything set up the way you like it (with macros, etc.) I would suggest starting with the SRD content, because manually adding the bits that are missing in the SRD will help you learn how Fantasy Grounds stores records and parses info.