The question is, what is the high price for? Are there cheaper alternatives for DC DC charging alternator to 12V (14.6V max) lithium? It seems to be a such a simple operation.
If the alternator is doing nothing else but charge the lithium battery then I have no idea why you would use a DC-DC for this or have a lead-acid stage. Instead rip out the lead-acid-optimised alternator regulator and replace it with one that produces the output voltage and current limit you want, with whatever temperature compensation you like.
I would start by deciding on the lithium charging algorithm, that then gives you voltages, currents, temperature compensation, time profiles, etc. That can be implemented in a small micro as the top-level behaviour, which provides inputs to two control loops (probably also in the micro, though these loops could also be implemented externally with opamps), one voltage and one current. The hardware has to provide suitably scaled voltage and current measurements, and the control loop adjusts the alternator field current servo the output to whatever it should be according to the charging profile.
One of the advantages of pulling the control loops into the micro is you can better optimise the transition behaviour between CC and CV modes, but it also just means fewer parts and easier to tweak.