I'd plan it out and setup some goals for yourself. Decide how much you can comfortably spend and work at sticking to your budget. And lastly, figure out your strategy. Is it buying domains for the lowest cost possible, or buying better names (potentially) for a little more?
In dollars, as you asked. I would say the following:
Beginner: To me, when I had about 300 domains, I just had them because I liked domains and I wasn't really selling them. I would just get offers here and there. So at this level I was more of a pre-beginner, and just a developer with a bunch of names.
I'd venture to say that most people with a $1,400-$1,700 average selling price is selling about 1.2-1.5% per year. There are definitely exceptions, but many times those people are not buying $10-$20 domains, and are instead buying at $100 or more and selling them quicker for the same price range. So, to me it seems like they are buying a higher tier domain, and selling it at a lower tier price range for a faster sale, and then they achieve 3-8% of domain turnover per year.
So depending on your strategy, whether it is a small number of domains or large number of domains, I would consider a beginner when you are reaching about 1 sale per month with an average of $1,400-$1,700 per sale. That's about $18,000 per year, less your domain registration expenses. In a recent partnership, I was able to get to about $24k per year in about 2 years with only a $10k initial investment. Granted, this was after I was already doing this for about 13 years. But I believe the first time around I was in a similar position after about 2-3 years of doing it from 2005-2008, but when I did it the first time I believe my costs were about $20-$30k, instead of $10k.
Midlevel - For me the next big milestone was $50k per year in sales. By this time, I redesigned my website (betternames.com) and offered a marketplace to customers. I would say that this is about Midlevel, as I've seen many other domain investors do the same around this level. For me, I believe I had about 2,400 domains around this time, yet several hundred domains I did not have listed for sale. At the other extreme, I've seen some investors have 100 domains or less and achieve the same sales, though I don't know how consistent this is for them.
Advanced - This really depends on how far you want to go and your strategy. For me, I'm trying to get sales to $250k per year within the next few years, but for others it may be much higher or lower than that.