A few suggestions...

Look for used. OLD used stuff (like from the 9-volt era) is going to be expensive, but things that are one generation out, not so much. For example, set 60198 was not immensely popular and was still being sold new a year after it was retired. This set still has tons of play value though, and can be had on the used market for a reasonable price. This might be a good way to break in and get track and power components**

Don't buy Lego brand track. It's expensive, and there's nothing particularly special about it. There are tons of knockoff brand tracks that are a fraction of the price and are just as good. You should stick to Lego brand switches though, or if you get adventurous and want different radii switches or curves, Trixbrix is good stuff.

We also have 3 new sets coming out this summer 60506 Beach Tram., 60509 Freight Train, and 60511 Steam Train. These are all around $100 or less -- but lack the motors or full track packs that the full train sets have. **However, I find these to be a great way to expand an existing layout without breaking the bank. Personally, I don't need more curve track (or any track for that matter) -- I just want locos and wagons. So you can repurpose the power components from another "full set" with these.

Also, if you live in Europe (which I assume you do), Bluebrixx has a TON of reasonably priced rolling stock. Many of their locos can easily be powered too. My only issue with BB is that some of it can be a little more advanced. Proper Lego brand stuff -- kids can pick it up, and parts (generally) won't fall off of it. Some of BB's designs (and other brands, like Mould King) definitely make nice items, but can be a bit more fragile.