Sorry Mattia, cheap modular synths do not exist.
I would take a look at semi-modulars, which can be used standalone and provide cv connectivity. Below you will find a selection of less expensive ones. You will find plenty of reviews on-line. In no particular order:
Korg MS-20 Mini
Doepfer Dark Energy II
Moog Mother-32 (slightly more expensive but with sequencer)
Hope this helps,
Mattia Ferrarini wrote: