PPC ads are sold auction style, and rates vary based on clicks, so the campaigns are optimized on an ongoing basis, taking into effect industry and seasonal trends.
Effectiveness of PPC campaigns tend to be valued slightly differently than display advertising. In this case of text ads, we are looking at CTR, or Click Through Rate, and the Average Position of the ad on results pages. The higher the position, the more effective the ad.
Ads are placed based on a combination of quality and bid, but an effective CTR can have a positive effect on a campaign, and a high Average Position is a good indicator of the perceived relevance of the ad & its budget.
Changes in market and optimization of the campaign can improve the Average Position of the ads. This is a good trend, and will improve the overall effectiveness of the relevant keywords.
It can also be likely that the competition in the market has changed, which will have a direct impact on the effectiveness of a $50 budget. This is exactly why one should focus our attention on converting the impressions we do garner into clicks, and maintaining a high quality score on these campaigns.
The effectiveness of the landing page is equally important to the ad and keywords in a PPC campaign, so it may be wise to continue updating the landing page on a regular basis. This will help not just the organic SEO value of the page, but also the quality of the campaign, and does this without increasing a budget at all.
Also, in an effort to improve the efficiency of the budget, campaigns can be targeted to a more narrow, DMA based geo-targeting. As always, this will be one maneuver in ongoing efforts to provide effective PPC services for you and you client.