Depends on where you boat, type of cruising envisioned, size of boat, boating experience.
And what gear you currently have on the boat.
I usually recommend to clients that they use the current electronics for a couple months and learn what is important to them and features desired. Unless current electronics do not work or unreliable.
It's always a good idea to have some experience when you shop for equipment.
Replacing everything at once is not always necessary.
Too many boaters feel that a full compliment of modern electronics is necessary for safe cruising.
Up until the 80's, a compass, depthsounder (usually a flasher) and VHF was all most boats under 60' was equipped with.
I don't disagree. I first started doing longer multi-day coastal runs 30 years ago. My boat had a Loran that would give raw Lat/Long readout that was then manually transcribed to a chart with its attendant possibility for human error so you cross referenced with dead reckoning, and when available, radar, depth sounder, anf ATON (including lighthouse flashes when coastal) . At the time Loran was a quantum leap over RDF.
A good friend has a 21 foot center console with a full Simrad system and 12 inch display. Probably $10k system. He uses the boat as a personal water taxi with near zero use for the Simrad system. Heck, it hasn't even been calibrated properly. Waste of money but sure looks nice.
But I interpreted OP question as a general budgeting question for a base system for a typical trawler or weekend cruising boat, not a minimalist one. I would hasten to add that a lot of fun can be had with a depth sounder and navionics running on a phone, just need to avoid weather, poor visibility periods, and abide by channel markers.
For a weekender, most people will find benefit in an autopilot interfaced to a chart plotter. While I am tempted to advocate for radar and AIS, I'll concede that this is more dependent on location and desire (plus, despite advances in auto-tune, I think many people do not know proper usage and interpretation). But a complete base system can be had for well under $10k depending on desired size of MFD and number of helm stations.
As far as expense, one item I didn't mention is carpentry. Older electronics had much larger footprints. Getting a reasonably attractive end product for an electronics upgrade can sometimes be difficult.
Peter