My upgrade from 4770F to 10700KF (preowned) was pretty damn cheap, total cost ~£210, because I sold my old parts for £200.
I think the upgrade from low spec DDR3 to decent DDR4 is worth upgrading for... and a doubling of CPU core count in my case.
My dad had some spare thermal paste, I used the same super old Hyper 212 Evo cooler, plus my brother's old mid tower case.
The cooler keeps the CPU (~4.7ghz all cores) at 80 degrees or under in stress testing (tested with hyperthreading off). Probably could run with HT on with a new cooler.
I'm glad I didn't spend more to get a 10900F, pretty sure the cooler couldn't handle it.
The build went pretty smoothly, except for fitting the hyper 212, it's always a pain to install because you need to get the holes correctly aligned on both sides of the mobo, before tightening the screws.
I'm testing to see the DDR4000mhz RAM will run at 4133mhz + XMP profile (reviewer managed this), no luck on 1.45v, so testing with 1.490v (HWinfo reports as 1.480v) - Is that a safe voltage for DDR4?