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Dave,
It can definitely be taken to an extreme, which is a whole lot of fun ;)
Short of that, depends on what you consider a relatively modest cost. Volvo put a lot of development into the injected B20s, and it's not cheap to improve on what they did. You have to do stuff that was considered too expensive for a production automobile.
The first step in an ES is to bring the motor up to '70-'71 specs. For the North American market beginning in '72, the motor was derated from 130 HP to 112 by lowering the compression, and a different head casting was used that had exhaust ports that don't flow as well as the earlier ones (possibly for smog reasons). You can retrofit the earlier head (say $500 for a rebuilt head installed), or rework the later one to outperform the earlier one -- but that involves expensive port work. Good porting is one of the nicest things you can do for a B20 motor, but it's not cheap.
The injection exhaust manifold is better than any of the cheaper headers on the market. There's very little you can just bolt on that improves it.
D-jet injection is inherently a high-performance system, but it's not very tunable -- it's really hard to make it work well with a hotter cam, for instance. That puts you into some sort of aftermarket injection like the Megasquirt or SDS system, which reuses a lot of the original components but still costs you $600+ if you do the work and programming yourself. Or you can go to the dual side-drafts, which is more expensive yet.
Realistically, if you want it to be reliable and not have driveability problems, you're looking at the $5000 range for a motor / intake / exhaust system that beats the original '70-'71 performance.
Here's one of mine:
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