The Search Lost IS (Budget) report tells you role that your budget is playing in your overall impression share (your impression share refers to the number of times your ads are running divided by the number of times they are eligible to run). Basically, if you see that you have a Search Lost IS (Budget) of 10%, this means that if you raised your budget you could capture an additional 10% of searchers. If you are doing a great job (which should always be the case!) of measuring the ROI of your keywords, then you should know exactly how much revenue you are losing by not increasing your budget in a particular instance. The counterpart of this report is the Search Lost IS (Rank).