Hive Commander Triple Threat
A few months ago I completed my Trygon. I posted it on a thread here.
http://www.librarium-online.com/forums/painting/105575-rudys-tyranid-trygon-wip.html
In my base I put a dead Ultramarine for reference, but it may not give you the exact size. I was told that a base the size of a typical CD/DVD should be fine. I made my base a bit larger, but the base was donated to me so I found it hard to complain.
I will say that the FW Trygon really looks great. I don't get to play with it much, because usually when I play with it, I use in Apocalypse games, and opponents are usually shooting at the Trygon with an Eldar Titan, Baneblade, or other super-heavy unit.
If you really are concerned with the budget, making one out of one or 2 Carnifexes may be a better choice for you, although I have no experience with this.
I know there was a rumor long ago that GW was thinking of making a plastic Trygon, but I also know that there was a rumor that they would also be working on a plastic SM Drop Pod. Neither of which I've seen, so I can't say for sure that they're going to be making one. But I thought it was important to note that a rumor of a plastic Trygon did exist before the release of the Apocalypse supplement.