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100€ Is realistic, $100 not so much.
Even then you'll have to mind the budget a bit in army selection but that opens up plenty of options.

my last (somewhat) objective-aimed list (didn't even have a secondary list): 4x bagh-mari + stephen rao (that will eventually be at least €45 together), spec-ops, palbot, 8pt mulebot, sierra dronbot, peacemaker, dragao. That'll be over €150 though I realize it wasn't exactly optimized to be cheap.
That gives a second mulebot (nasty; not something you'll often use!) and a second dron-bot for 'variation', which isn't all that much.
Now that has a TAG and three 40mm based remotes so not the cheapest list possible but it's by no means out there; the one CA list I'm currently painting up is more expensive (two boxes of drones, two ika/ime blister and a box of hungries before getting to the real meat of the list might have something to do with that though so this is not the best example).

Even then though, I don't think I know any infinity player around here who stuck with a single 300 pt list for any length of time and if you want some real variation (an extra HI model, some cool sniper, etc) that's not going to come from the 'extras' in your otherwise already planned purchases.

$150 is fine as a starting budget, keeping in mind you'll be spending a few dollars here and there in the months after. Any lower and you're probably hamstringing yourself.

Psychotic Storm, there's a reason you brought three different lists, not just one basic 300pt be all end all list