Sorry but there is no average, rates are dependent on may variables, contact a few agents for quotes.
Are you talking about a magna or sabre?
This bike takes 3.7 quarts of engine oil. Measure the oil level with the bike in upright position, not on the side stand. Change the oil filter every 5.000 km or at least once a year, if driven less.
10w40 will do just fine.
check the fuse?
Ngk dpr8ea-9
In the back of any repair manual.
The Si carries an 11.7 gal tank.
According to the Honda 1984 XR500R Owner's Guide, the engine oil capacity is 2.1 US quarts after draining.
sorry for my bad inglish,first to all the Honda magna use a 7/8 handlebars and the OEM grips are 5"5/16 long iam using an aftermarcket grips on my 1985 magna so if you find a 7/8 hole grips that grips are your new ones ...lunaluna305 from Costa Rica
Use for example NGK DPR8EIX-9 or NGK DPR8EA-9 spark plugs.
It may not be possible to download an entire manual, but you could look up specific problems on the Internet and find answers.