The merchant steamship is arguably the most grievously neglected subject among the mainstream plastic kit manufacturers. It's just about possible to count all the plastic merchant steamships ever released on your fingers and toes combined. (Well, not quite possible - but almost.)
To my knowledge there's been only one Lusitania kit: the big 1/350 that was originally released by Gunze Sangyo back in the late seventies (and re-released by several other companies since). You're right: it's quite expensive - though a basically good kit in most respects (caveat: I haven't built it).
The Lusitania had a near-sister, the R.M.S. Mauretania. (The most visible difference was in the ventilators on the upper deck; the Mauretania had big, traditional "cowl" vents, while the Lusitania had more unusual - and somewhat simpler - vents with hinged tops. Any decent set of plans will show the difference.) For a long time Airfix made a Mauretania on 1/600 scale - a really nice kit, considering its age and scale. Converting that kit to the Lusitania wouldn't be terribly difficult - and a set of Gold Medal Models photo-etched detail parts (the "1/600 Merchant Ship Set" would be the one to get) could really turn it into a showstopper.
Unfortunately Airfix underwent a big reorganization recently (following near-bankruptcy). The good old Airfix kits from previous decades have been gradually reappearing on the market. Unfortunately that Mauretania hasn't come back yet. I imagine it will eventually; in the mean time I don't know how hard it would be to find on e-bay or someplace similar.
Sorry to be so uninspiring, but as far as the plastic kit industry is concerned the ocean liner enthusiast is really out of luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.