I think those articles are a good investment. Consider this: most webmasters try to grow traffic by paying for advertising. How much PERMANENT increase in traffic will a hundred and fifty bucks buy? A little, possibly a measurable amount, but not nearly as much as a half-dozen relevant and keyword-rich articles.
My website is a content site with a bunch of articles I wrote myself. I recently commissioned an article, and plan to commission more as I'm able to afford them.
I DO spend some money on advertising, but only about $50 to $100 a month. It is very carefully targeted, and aimed at bringing in visitors who are likely to link my articles rather than simply pumping raw traffic. I think using advertising to drive traffic to the adsense ads on your own pages is a very poor investment, and unlikely to even break even except in very unusual cases.
When I'm able to quit my job for adsense (which, it turns out, is likely to be soon) I expect that I could publish a new article once a week or so. That should greatly improve my repeat visitors, who so far have seen new content only sporadically.
I think that doing so will result in more ad revenue than I need for my living and business expenses. Basically my plan is to reinvest any profit back into the website. A small portion of that will be ads I pay for. The larger portion will be to commission articles from other writers. I think that by doing so, and regularly working with a pool of a dozen or so freelancers, I think I should be able to publish a new article every day.
THAT is the key to retaining your traffic. If a visitor can see new content EVERY SINGLE DAY, and you are able to pay for the content with your ad revenue, you basically have gotten yourself a license to print money.
I realize you're a long way from that goal, but you've made a significant start, with your decision to invest your ad revenue back into your site rather than spending it on living expenses.
I have one more word of advice: learn from experience. Only YOU can figure out what strategies and tactics will work for YOUR website. You can find a lot of good advice at webmasterworld, but you will find some advice works well for you and some doesn't. The reason is not that some advice is poor, but that every website is as unique as the individual who is its webmaster. Some advice simply won't apply, or not be worth the effort to implement. Others will be like nuggets of gold.
Whenever you make a significant change to your site, or pay for advertising or what have you, keep an eye on your ad performance report and, importantly, your web server log files. Use a log file analyzer to track your website statistics. I use Analog (http://analog.cx/) There are many others, the important thing is to learn how to use one well and to use it regularly.
Browse the forums to look for tips that have already been posted, and keep asking good questions such as the one I am replying to.