Google is great at providing help and information on the services available. If you have a good ROI then it is likely that you will increase your spending and that Google will be presenting better quality content to it's users.
If you were to ask the same question about quality and ROI's to Yahoo, Bing and just about any other search engine, they will all try to help you improve your roi and thus increase your spending with them.
Google has produced a basic guide for improving your ROI. This document is well worth reading.
Here is a sample but please read direct from Google and understand how and what ROI is.
Return on investment (known as ROI) is the ratio of the cost of advertising relative to the profit generated from conversions such as sales or leads. Your ROI indicates the value to your business gained in return for the cost of your ad campaign. Watch the 'Intro to ROI' video to learn about ROI and why it's important to your AdWords account.
Although exact measurement is nearly impossible, you can help assess the ROI of your campaign by using these calculations: take your revenue from sales, subtract your advertising costs, then divide by your total advertising costs.
(Revenue - Cost) / Cost
For example, say you want to drive users to your website in order to generate sales for your gadgets, which results in a profit of £10 per gadget sold. You invest $1000 in your ad campaign for the past week, and as a result, you sold 130 gadgets. Your revenue from sales or total profit is £1300 (£10 profit multiplied by 130 units). Thus, you subtract the £1000 cost of your ad campaign from your £1300 profit to receive £300. The £300 amount is profit returning to you as a result of your initial £1000 advertising investment. Your ROI, expressed as a percentage of your initial investment, would be 30% (300/1000, multiplied by 100).
Read about Return on Investment at Google.