Yes, I remember in Property Ladder about six years ago, where a couple were restoring an Art Deco house, they spent about £25,000 (bearing in mind 'local money', and a while ago) replacing original steels with Crittal reproductions. It's no wonder you got the price you did, Tighin: steel frames from the UK (and especially with today's metal prices) are notoriously expensive in Ireland.
Slimlite is certainly an option, but the key point is the structural integrity of the existing steel members. If they are 1930s, they will not be galvanised, hence I imagine they're not in as good condition as late 1950s frames, so they will have to be carefully assessed. Slimlite also cannot be installed using the existing single-glazed metal clip and putty system - rather it is siliconed in position, allowed to set, and is then puttyed over to maintain the historic profile - this is usually achievable. The cold bridge of the steel sections is unavoidable - something that has been ironed out with a thermal break incorporated in new steel frames - but in the context of the overall expanse of glazing it is not considerable. It is worth speaking to a specialist on the potential for condensation issues at junctions. Lambstounge Ltd. is the only supplier of Slimlite glazing in Ireland and is definitely worth talking to.
Personally, I would be reluctant to install any expensive glazing in existing steel frames unless they were fully assessed by a metalworker for their integrity. Even then, 1930s glass has characterful imperfections - visible even as late as the 1950s - which can be worth retaining. Generally speaking, an overhaul of existing steel frames coupled with tailor-made, internal timber secondary glazing featuring sections that respect the pattern of the originals, is an option I would aim for. The internal should be single-glazed, not double, for reasons of thermal necessity, cost, and inherent flexibility of delicate timber sections.