Not sure if it's still available but a while back valkyrie_76 had a MISB Justitoys Trailer going for $75 (looks like no one reserved it in the thread at least).

I haven't seen either but i've heard the quality on the Justitoys trailer is up to Takara standard, but it doesn't feature Roller (neither does MP04 trailer), and doesn't transform/fully open (MP04 does).

Here's a good pic of Justitoys Trailer ($90 + shipping from Transfigure Toys)

And here's MP04's trailer on TakaraTomy's website