Sorry to take so long to get back to you on this, had last day with consultant today and my head is still spinning!
Saker I haven't done an ABA programme with my dd, but have worked part time, (currently only one morning) as an ABA therapist for about the last 18 months. I have used some of the ABA behavioural approaches with dd under the guidance of the ABA consultant I work with. However I do have a rough idea of the annual cost of ABA if being self funded and the RDI assessment and set up of the programme has cost us less than 10% of that.
Davros my experience of ABA is very different to a family who have run a programme and it being so much a part of their lives, also I am only just starting out on "my RDI journey" so am certainly no expert on either interventions So from that perspective my reply to your answer would be yes I do think RDI could be done on it's own to treat autism, but ABA is good at teaching skills like matching/imitating/categories/features/functions etc. RDI would then be able to teach the child to use those skills in a social situation.
The RDI consultant (who has done ABA/teach etc)was asked the same question today by a mum who had run ABA for several years, he described ABA as being more useful to treat a co occurent disorder such as learning difficulties, he feels ABA is about improving the childs intelligence at whatever level, and not about addressing the autism. The danger of teaching skills is that the child is storing the information but not recalling on it to use in a social interaction.
Eg dd has stored many facial expressions/emotions that have been taught rotely but does not know how to generalise them enough into real life social interactions.
I can also see how for some families it would be really hard to "let go" of ABA. I think it very much depends also on how holisitc the ABA programme is, and at the end of the day a parent has to choose the intervention that they feel is right for them and their child.