I really like the sound of the Access Virus TI, however, they're out of my price range. Most synthesizers sound thin, weak, or retro. I am looking for something with a full, very pleasant, and modern sound, similar to the Virus TI. I would also like it to be very flexible. Are there any synthesizers that could serve this purpose?
Feature-wise, I'm sure there are plenty, however if you're chasing the "sound" of a particular instrument, there really isn't going to be any substitute.
That said, if you think other synthesizers sound "thin, weak, or retro" (not terms I normally associate with synths) than what you may be liking about the Virus is simply that it employs a variety of synthesis techniques - subtractive, FM, wavetable, and phase modulation. A lot of synthesizers focus on one technique, with that technique's characteristics. So, another synth with a similar variety of timbral techniques might be what you're after. I have hunch, perhaps unfounded, that it's the wavetable technique that most interests you since you used words like "full" and "modern."
If you're interested in wavetable capable hardware, look into machines such as Waldorf's Blofeld. Desktop or rack modules are, in general, less expensive than keyboards. If you're alright with software, there are a huge variety of everything-under-the-sun synthesizers such as Native Instruments' Massive which is quite popular at the moment.
There isn't really an authoritative answer to this question (i.e. there's no standard "poor man's Virus" and of course it's down to your taste anyway) but synths similar to those two are where I would start looking.