Honestly, you may be thinking in the wrong direction. Let me explain:
Why you may not actually want a more pricey tripod
More expensive tripods tend to be made of lighter materials (carbon fiber is a favorite, and it's what yours is made of). This is for the benefit of photojournalists (etc) that have to carry several cameras, lenses, and other gear. Every pound they can avoid is another thing they can carry.
What you probably really want to do
For your purposes, assuming you are not hiking long distances or carrying a plethora of accoutrements, you may actually want to find a cheaper tripod or one around the same price point that is made of heavier material.
A (perhaps more economical) option
Alternately, you can buy slings that attach between the legs for accessories. Instead of filters and lenspens though, you can just drop some rocks in it from the surrounding nature. Another (almost identical) option is sand bags. Giottos makes sand bags that velcro to your tripod legs. Anything that increases the weight of your support will also increase the stability (up to a point, then it'll snap in two).
Your problem is almost definitely caused by wind, as no other forces can really be acting on your setup. I'm assuming, of course, that you are either using a remote release or you are shooting with a drive mode that implements a shutter timer. If this is not the case, you may actually be causing this minute movement when you release the shutter.
Remember that any movement at all will botch a long exposure if your subject is comparatively well lit (like stars, in your case). So removing your finger from the release after triggering the shutter will almost definitely be visible in the final image.
You don't say what body and lens you are using, but it would have to be pretty monstrous to exceed 17 lbs (the capacity of your tripod), so I don't think that's your issue even if your QR plate isn't properly balanced.
One last thing to check
There is one possibility I haven't covered. That is your tripod feet. Most tripods have little rubber feet on them, but little rubber feet aren't always ideal for gripping terrain. For this reason you may be able to order other feet with little spikes on them that will grip in loose earth and softer mediums. Having brought that up, I'll say that I can't imagine this being your issue if the vibrations in your exposures are consistent on different surfaces.