1 AU is 149597870700 meters. This is much more information than typically needed, so 150 million kilometers would be precise enough in most situations.
Does spreadsheets have a function to handle this? (I use Google Spreadsheet, LibraOffice and Excel)
Currently I do it by dividing the number to 14,9597870700 and round it. Then multiply up again.
log10(149597870700) = 11,18
floor(11,18) = 11
11+1-2 = 10 # two significant digits
149597870700/power(10,10) = 14,9597870700
round(14,9597870700) = 15
15*power(10,10) = 150,000,000
A combination of LOG10() and ROUND() will achieve this. I found a reference here that gives the Excel formula
where number is the number you want to round and digits is the required number of significant digits.
I don't have Excel, but an equivalent formula in a different spreadsheet worked as intended.