There's a lot of information here about why chains wear, how long they might last, how to tell when they need replacing, and what happens if you don't when they need it.
- 1000 km (cross-country, or all-weather abuse)
- 3-5000 km (well-maintained derailer chains)
- more than 6,000 km for perfectly groomed high-quality chains, single-gear, or hub-gear chains (preferably with a full cover chain guard)
So if you're doing 800 km/month, then you ought to replace chains maybe monthly if you don't clean and lube them every week, otherwise 2-3 times/year.
There are some conversations here, here, and here about how often a cassette (i.e. rear derailleur sprockets) might need replacing.
The consensus there seems to be that a cassette must be changed approximately once for every 2 to 5 chains; and depending on how expensive the cassette is, if you're using a relatively expensive cassette then change the chains more often (to protect the cassette from being abused by a worn chain).
Other estimates suggested 10-20,000 km for a cassette.
So if you're changing the chains 2-3 times/year then expect to change the cassette once/year?
And a cassette costs maybe $80 for the part (excluding labour; if it needs changing, the LBS will probably change it when they change the chain).
How much maintenance (not just what type of maintenance, but how much) does a cheaper bike require?
Don't get a bike that's too cheap (a 'BSO'): no amount of tuning will be enough.
A maintenance schedule will include:
Clean the chain weekly (otherwise, at 40 km/day, expect to replace it and maybe the cassette every month or two).
Check the tire pressures weekly.
Adjust the brakes and gears monthly (either yourself, or at a bike store)
Change tires whenever necessary
Have an in-store tune-up 2 or 3 times a year: during which they may change the chain, cassette, brake pads, and tires.
Cost of thrice-yearly service, and new parts (pads, chain, and cassette) might be $600/year (or more if you don't do your own weekly chain cleaning, and monthly brake and gear adjustments).
And/or, how much longer will more expensive components last?
An internal-geared hub should be adjusted/serviced once, after it's installed.
A 7-speed internal-geared hub from Shimano might last 3 years or so, and cost $500 to replace; it's grease-filled.
An internal-geared hub from Rohloff lasts indefinitely, but costs $1700+ to buy; it's filled with oil instead of grease (change the oil once a year).
With an internal-geared hub (unlike with derailleurs) you can fit a chain-guard around the chain: in which case the chain might last twice as long, and want cleaning every two months instead of every week.
If you have rim brakes instead of disk brakes, then you may need to replace the rims (i.e. the wheels) every two years or so. Mechanical disc are cheaper to own than hydraulic disc