Believe me, you don't want to go cheap. Your wallet wants to, but that's just not how modulars work. If you absolutely have to have a full synth right now but it has be cheap, go DIY.

Anyway, if I may suggest a few things:

1) Two oscillators (one with LFO mode), a mixer, and a filter are pretty much the basic modules. That allows you to play with both 'coasts' of synthesis and other wierdness. After those you'd want to add an EG, preferably a MATHS, but if not the Pittsburgh ADSR is quite good.

2) The mix mult is a good choice for mixer as you'll really only be using 3 channels most of the time and it has two mults - always useful even with stacables.

3) I'd recommend spending the extra $20 and getting the dixie over the A-110. Far more useful. Two of those and you can sate the oscillator 'requirement'.

4) I get that you may want an actual sequencer, but if that's not required you can use a MIDI interface instead. Same idea, much more useful. Cheaper, too.

5) for a filter, I recommend the Pittsburgh filter. It's very clean, has the four main filter types, and is easy to use with a lot of features not found on other filters.

If you decide to go along with these suggestions (which you don't have to), You'd end up with something like this: http://imgur.com/dhN4l4Y

Oh and btw, it's totally fine to build your modular slowly over time. Start with a case/rack and power supply (I recommend the tip top audio happy ending kit), then add an oscillator. Then either the filter or midi interface. Then the mixer. Then EG. You get the idea.

Anyway, that's just my suggestion. You probably know what you want your synth to do and it's probably a completely different idea from mine. You do you :)