Simply put, the price the car is advertised at. So when they say "$39.95 per day plus taxes and fees", the base rate is $39.95. Here's an example:
Pick-up VANCOUVER INTL AIRPORT, BC,CA
Feb 18, 2010 09:00 AM
Return Same as pickup
Feb 19, 2010 09:00 AM
Car Class: Dodge Charger or similar
Rate: $50.35/day
Total Base Rate (CAD): $50.35
Rental Time Charge $50.35
GOODS SERV TAX (5%) $2.68
LOCATION SURCHARGE (17.75%) $9.53
CANADA PROVINCE TAX (7%) $4.43
ROAD TAX ($1.50 per day) $1.50
VEHICLE FEE ($1.84 per day) $1.84
Total Estimated Mandatory Charges (CAD) $70.33
Estimated Grand Total (CAD) $70.33
Say if your rental was for 2 days at this rate, the "Rental Time Charge" would be 2 days @ $50.35, or $100.70. So the car is advertised as being $50.35, but the real total in this case is $70.33. Hey, we have to pay for our Olympics somehow...