The costliest mistake when setting up a modpack server is buying on price instead of what the pack actually needs. Here are the most played ones with their real data — version, loader and how many mods they carry — and the gigabytes you need depending on how many of you there are.
The modpacks aren't downloaded from here. You install them from
your host's panel (nearly all have one-click installers) or from their official
page. What you get from this site is the settings pack: a
server.properties tuned for that pack, the startup arguments that
stop the stutter, and step-by-step instructions.
Proper Pokémon in Minecraft: gyms, leaders, a league and progression, built on Cobblemon. The one people ask for most to play with friends.
RAM you'll need
Minecraft with everything on: structures, mobs, dungeons and improvements everywhere, without turning it into another game. Also on Forge (BMC4).
RAM you'll need
Adventure with magic, classes and quests. 555 mods: one of the heaviest there is, and it shows on the hosting bill.
RAM you'll need
Classic Pixelmon, with its own Pokémon system. Few files because it's one single mod, but a huge one: count it as a big modpack.
RAM you'll need
The 100-days challenge with zombies, guns and waves. The most downloaded of them all, and one of the best in a group.
RAM you'll need
Adventure with slimes, dungeons and bosses, on the newest version. Balanced: plenty to do without needing a monster machine.
RAM you'll need
Cave horror: creatures, darkness and jump scares. The best one for a streaming night with friends.
RAM you'll need
The Create pack: machines, trains and huge factories. Here RAM matters less than the CPU, because machines run every tick.
RAM you'll need
Every mod adds its blocks, items and recipes, and all of that sits in memory while the server is up. That's why a 500-mod pack with four friends needs more gigabytes than vanilla with thirty people. The numbers above come from the same maths as the calculator on the home page: a tier by player count, multiplied by how heavy the pack is.
And one warning that saves you money: more RAM doesn't make it faster. What actually moves a modpack is single-core CPU speed, because Minecraft runs almost everything on one thread. That's why the comparison lists each host's CPU.
Tell me how many of you there are and which pack you run, and I'll tell you the exact gigabytes and what it costs on each host, with the entry discount and the normal rate kept apart.