I'm with Vas - depends what you mean in terms of modifications and afordable. The ones I did off the bat:

2003 330Ci ZSP
  • Piezo alarm mod (my car didn't come with an alarm system) - ~$5
  • Angel eyes wired to my interior lights - $60
  • CDV delete - $0
  • ZHP shift knob - $65

I believe that's all I did for that car before it met its untimely end with an antelope at speed.

2005 330Ci ZHP
  • BMW Performance grills - $80 (while still cheap)
  • CDV delete (actually it's with a UUC connector) - $0
  • Cell phone mount - $40
  • Coding things like one touch windows - $30 (I can't remember how much it cost to by the BMW Scanner from a member here)
  • Front strut tower reinforcement plates - Can't remember the price but I know they're cheap, especially if you do the labor yourself.
  • eBay LED license plate lights and reverse lights - $20? They were cheap and I prefer the LED look since the whole of the rear is LED
  • E9x rearview autodimming mirror with garage door opener and compass - Mine didn't come with the Premium Package and I wanted the autodim feature.