First of all, if you're hoping for a server with 100+ players online at daytime, it's much more difficult and rare than you'd think. I feel like there's not so many people around anymore that's looking for OT's to play. I guess many have just settled for the ones out already or OT is simply dying, I'm not sure.
Feels like the only one that's somewhat populated are the ones that has either spoofed their server at some point/points or the owner is well-known or it's some kind of old server that is resetting. If people feel differently then I'm glad to hear your opinion, this is just what I feel is going on.
If you still feel like this doesn't matter then I'd say the cost is completely based on how much work(customisation or how incomplete the server you download is).
To deduce this you need to either find all info about the server you download(most likely at the servers github) and/or test everything so that it's working fine without bugs and no missing parts of the systems. I'd go with the newest TFS if I were you, I am not aware of how complete it is as a download & run but it's probably your best option.
Then you'll have to compile a list of all the things that need to be done and then you can send that list for people to look on so they can estimate an amount they'd find themselves pleased with. Or you find a person who already knows what needs to be done and discuss with that person.
If you wish to add your own customisation by yourself,
then a download & run server is easy and shouldn't cost you more than 30$ imo for someone to set it up for you on a hosting machine.
If you're going for OVH host which is the most popular one amongst OTS then a good host(dedicated) would go for 60-100$ monthly and like 70-120$ setup fee depending on what you choose, I'm sure there are cheaper options but that's what I know.
I hope this was somewhat insightful or helpful to you sir, it's not easy to guess an amount if someone doesn't know the workload beforehand, I also don't wanna write more than I already have as it already is too long. It's not a small topic for sure and I'm not here to write a book.