Personally, I like my rear wiper.. it's useful as hell in the rain.. but that's besides the point here..
Mentioned earlier was to upgrade your exhaust. Unfortunately to most, that's not exactly the "least" expensive upgrade. There are quite a few things you could do prior to that, unless you're talking about JUST a muffler tip, which would then be inexpensive..
I would probably go with springs before exhaust, at least for modifications sake. Springs will run you about $200-$300 for decent ones, depending on where you get them, where decent exhausts will run you about $500'ish. This price is including installation (if you choose not to do it yourself)
If you have some kind of aftermarket rims (and not steelies with hubcaps), you can paint your calipers ($10 for a can of high-temp caliper paint).
You can get strut bars for your car. I got a set of Mugen replicas for $40 on ebay, brand new (and they're good).
You can get your windows tinted. I happened to find a place that warranties your tint for a lifetime, all windows excluding the moonroof for $180... using Llumar tints (good quality!).
Rear seat removal is a good one for minor weight reduction, and that's free!
These are just a few things you can work on in the meantime. Those prices are relatively adequate for what you could consider "mods," whether visually or for the sake of performance. I'd say most people would be able to perform those mods without having to dig too far into their savings, even if they do bust their asses at a restaurant. =)
Anyhow, welcome to Team-Integra. If you give this place a chance, this place will give you the world (for your car). hehe..
Don't lower your car unless you have the money to get it aligned. $50 for alignment and possibly $100.00 for camber kit needs to be set aside, it may not be used but you NEED to have it in case. Otherwise, you'll eat your tires and spend $300-$400 more for them also.
Inexpensive and reliable would be cold air intake (K&N ebay for $60.00 shipped), short shifter (A++), tinted windows, muffler (not full exhst), Mobil 1 oil / Tranny fluid (A+++)
Cheap is not reliable, reliable is not cheap. DoNT cut corners with the suspension. There's a reason why those things are expensive.
and also maybe clear or black front bumper lights (the front blinkers). I got a good pair on ebay for $23 shipped! nothing but normal screws, take ya 5 mins and can make a great difference, enjoy the mods! tegs are very fun cars to have!
This is hillarious reading all these replies =) So many people think there is a set way to modify a car, and certain "mods" that are a must (IE: your car isn't cool unless it's lowered and has an exhaust). .
Tim, look in the common (or sticky) topics for the area you would like to improve (or customize) on your teg (we have different forums under the "Forum" link which cover each area). A $400 budget means nothing. What do you want to change on your Teg? There are as many options as there are members on this website (which is over 50,000 btw).
Most "inexpensive" modification involve mostly DIY work, which we happen to have a whole forum dedicated to that, here. There should be plenty of idea's there which you can read thru.