Pricing is a very painful subject, part of the problem is that BT miniatures aren't exactly common at (web)stores, especially the more popular ones. So often shipping is added to the total cost, for those of us that life on other continents things get really expensive with even more expensive shipping, taxes, and import fees.
But if you think BT miniatures have gotten expensive, you might want to look at the biggest pony in the park, GW. In 1988 they produced one of their first 40k plastic kits 30 plastic Space Marines for 9.99GBP, 23 years later they ask 23.00GBP for 10 plastic Space Marines. Almost seven times more expensive then 23 years ago. What I do have to give GW is that their plastic kits have gotten more detailed over the years.
Also keep in mind that IWM wasn't around 23 years ago, there was Ral Partha, eventually owned by FASA. Now IWM pretty much only makes licensed properties in primarily metal. I suspect that they have little to no knowledge/skill in producing plastic, I believe they even outsource their resin casting. They are primarily metal casters and are in pursuit to cast more metal, not move on to new/other materials, that's not their business. Not to mention that they aren't really developing their own properties, they are dependant on CGL/Topps for that and investing heavily in for example plastics (a long time investment) is very risky. Especially in the Mech market they have little to no competition, so there is very little business motivation to stife for excellence
If you want (high quality) plastic Mechs, that's CGL's boat you have to rock. As for "it's not possible to do the BT range in plastic", not if you want to do the entire range at one time it isn't, but I suspect that if some thought was put into the project you could get a long way with a few releases a year. Separate plastic hex bases, 10-12 Mechs per set (looking at the GW frames that would probably fit with the new Thor/Loki quality in mind), they could price that company of Mechs what you now pay for a lance. If you could get that quality/pricepoint into (web)gamestores on a global scale, demand would probably be enough to generate a hefty profit. But like I said, that's CGL's boat, if the unavailability of the starterset and now the core rulebook is any indication, they aren't organized enough to pull of a well timed plastic release schedule.
As a side note, I just bought an 'unseen Warhammer and Longbow' in plastic, 5" tall for $6.73 a piece. Perfect for 15mm/1:100 Battletech. They are a pain to assemble well, but the cheaper Gundam Bandai snapfit kits are a joy to put together. The 1:144 kits fit well (size wise) with the 15mm/1:100 Mechs. I bought a quad for $3.85 and it has six points of articulation, if you want a more detailed kit you can get one for around $13.47 with more weapon options, way more points of articulation, more details, different colored plastic for different parts, etc, they even transform if they have that ability (LAMs). They now even have a range of such high detailed kits that they have a highly detailed internal structure (kits are around $21.04).
