It depends on what time of the year it is. I get product-happy around the holidays.
Normally because I have more money to spend. And when it's a change in the seasons. I have to adjust my products when the seasons changes.
But since I normally stick with my stapes, I spend less.
Back in the day, I used to get my hair professionally styled every 2 weeks. So I spent about $100-120 a month on hair. More when I needed a touch up. This doesn't include the products I used to maintain a short do, all the way up to growing my short cut out.
Since I haven't been to a salon in years, I spend less than a quarter of that a month. My conditioner lasts me about 3-4 months(33.8 oz at $16), my shampoo lasts about 6-9 months(33.8 oz at $15), and all of my gels last more than 4 months(all under $6 bucks).
So, I think I spend WAY less as a natural(now) than when I was relaxed. Now, when I first became completely natural, I experimented much more to figure out what products worked best for me.