For sure, Hunter, Neshek and Nicasio could all be gone next season. And Arrieta is gone after 2020 unless he opts out this year (which is very unlikely). Neshek is the only one of the three relievers with a 2020 option and he's not that expensive (only 5.75 million this year, option is for 7).
I'm sure if they let them all go they will sign another bullpen vet for 2020 though. You always need a mix, and Seranthony and Neris have not emerged as surefire options (plus, what's a surefire option, Ken Giles? Brad Lidge?). Morgan's emergence is nice.
Like Andy said, it's either money or prospects. I suspect there will still be prospects going in July, probably for a younger controllable bullpen arm with some experience, rather than a FA to-be.
Back in '08-'10 we paid $4-5 million for 7th-8th inning guys like Romero and Madson, what Hunter and Neshek make is no worse now (while Robertson is being paid like a closer).