I agree with Steve Maxwell, if you invest in equipment specifically for the "90-size" models, then this may not be useable for the 2M models. On the other hand, a "large 40-size" model can fly quite well on a 5s pack (5-6 Ah capacity) i.e. one of the two packs used for 2M models. Then you can later use two packs in series when you go to 2M models. You would require a new motor and regulator, but this is fairly (???) cheap compared to the batteries.
Invest in a charger that can handle up to 10s li-pos, and use a balancer.

Back to the original question:
Is there any signficant cost advantage in electrifying the smaller pattern planes?

Do you mean cost advantage compared to glow?

Any electric model will be more expensive than an equivalent glow model. More so the larger the model is, but even electric 40-size models are more expensive than the equivalent glow model. (Providing you actually include all expenses required for practical use.)

Magne