I would not say it is more expensive in general. What I mean is, I discovered about a year ago that I had curly (wavy) hair. That was only after going no poo. I did not take care of my hair before then like I do now. And my hair was never has nice looking or as healthy. So, for a straighty to take good care of their hair I don't think costs any more than a curly.

As far as what I spend...except for my LI and gel, I have to use online boutique type stuff. My hair hates all onground conditioners locally available to me. (And it likes only pricey onground stuff...seriously.) Aside from the RO, I haven't tracked how long stuff lasts me but I am guessing it is all about 6 weeks. That comes out to about $35-$40/mo. Holy crap. I hope my husband never sees that... haha
remember, with all advice or suggestions for your hair, YMMV!!
My hair:
2b wavy, medium texture, low porosity, med-high density. Protein finicky.
My routine:
Cleansing: DCNP, every 4-5 days (occasionally use CJ cleansers).
RO: CJSC, occasionally use JCDC
LI: KCKT or CJSL
Gel: KCCC, CJCIAB for 2nd day hair (sometimes third)

Heat is my friend: warm water rinses, steam caps, and diffuse dry. I love CLU cloths and my Denman.