Actually I can envisage a simple node that does things like this... but of course the instant reaction is scope creep... I would want simple add, subtract, multiply, divide etc of the "payload" by a constant, (eg see Steve's buffer parser) or flow/global property... oh and then combinations like multiply and add etc (degC to degF etc). - oh then maybe AND, OR, XOR - etc etc... Not saying it can't be done (probably should be) - - but needs some design work first - and then indeed should it be part of the change node ? (see other recent discussions on change node) - or a new numeric only calculator node ?
And maybe a conversions node? I could see that would be REALLY useful.
These nodes would also help take Node-RED into improving some areas that it is weak on compared to some other visual tools as well which would be nice. One day, it would be wonderful to be able to use Node-RED to do ETL tasks that people currently switch to Python for.
There was a recent conversation on slack about adding a local context to the change node, perfect for things like keeping a counter that can be modified by any expression syntax... perhaps it's time to rethink that as it's a general purpose solution?