There are lots of mods for the R170 but none of them are cheap.
Pretty much agree, but I guess the OP should define what is "cheap" in his mind.
Some almost free "mods" are: Put you car on a weight reduction diet: get rid of spare tire, tools and any air compressor (and all other junk clanking around in your trunk) and use AAA or similar if you get a flat. Not everyone will like not having a spare, but since SLK32's dont come with a spare tire.... Buy a 14 lb battery for about $80 and save another 30 lbs.
I have seen ecu tunes for the AMG 32 motor as low as $400 now, not sure on pricing for SLK230 tunes and if the OP would consider that "cheap." Same comment on pulley kits to spin the supercharger faster and make more boost, some might think the $/HP gained is good for a mod like that and some might think it isn't cheap.
Bilstein shocks can probably improve your handling quite a bit if you have the standard suspension for not much more than $100 per corner plus installation. Again, not sure if that fits the OP's definition.
Also, unless you have done it recently, change your air filter! Don't want to restart the debate on OEM paper vs K&N or Greens, but a dirty air filter on a forced induction engine will cause a noticable loss of power. Make sure you have a fresh clean air filter.