These days unless the mod is to correct an OEM shortcoming, I tend not to do it.

For example, my little Saab wagon wallowed like a pig when cornering. Over-steered like crazy in the wet. On went front and rear strut tower braces, steel end-links, and a rear sway bar from a WRX. Result was more secure handling in low traction situations.

I'll also upgrade a weak OEM audio system to an after-market system.

The problem with modding is that you're always putting your car out of balance. You improve your intake, so then you have to improve your exhaust. Now you're moving even faster, so you have to improve your brakes. It goes on and on.

Another detriment to modding is that by the time you're done, it adds nothing to the resale value to your car. A car with a KBB used value of $10K that has had $7K of after market mods is still only worth $10K.