How will my overall ROI increase, by increasing my budget in Google?

Scenario #1:

If you’re averaging an average cost per click (CPC) of $2.40, and running a $5/day budget, you will receive about 2 clicks per day – or 60 clicks per month. With an average of 1 in 5 clicks resulting in a call to your shop, you’ll receive about 12 calls to the shop every month. If you can convert just half of these calls into business, that would equal about 6 additional cars per month. Now multiply 6 by your average ticket price. If you average a ticket of $400, then you’ve made $2,400 from Adwords traffic alone.

Scenario #2:

Alternatively, if you’re running a budget of $20/day, with the same $2.40 average CPC, you would receive about 8 clicks per day (or 240 clicks per month). Again, with a 20% call rate, this would total 48 calls to the shop per month. If you can convert half of these calls into business, that would equal 24 additional cars per month, or $108,000 in additional revenue.