Hi B.: Noticed that my breastmilk production reduced dramatically at 5-6 months, and then little by little after that. So, I mixed formula and breastmilk together (Iwas pumping). That way she still got all the good antibodies from me and fat and calories from the formula. She self-weened nursing at 9-10 months... but perhaps she just found a bottle of formula was easier than sucking on a dry well-?!
I told this to a girlfriend of mine: If you plan on eventually using to the cheaper, generic brands (that claim to be similar to Enfamil, Carnation or Similac) then start out on those. Don't try to switch later or you might find that the baby refuses it... and then your stuck with a cannister of unwanted formula. I've read numerous articles that say all the brands are nutrionally the same (government requirement), however speaking from my experiences with my daughter refusing my attempts to switch, the generic brands might be a touch grittier.
Formula is still expensive, regardless of whether you use the generic or name brand stuff. A cannister of name brand runs about $18-20 each; generic is around $12. Gear her tastes for the cheap stuff... At her peak of formula consumption, my daughter consumed one cannister of powder every 4 days. When she insisted on the pre-made name brand formula, we were spending about $300 per month.
Of course, if you find that your grandbaby shows allergies to cows milk, then you might have to exclusively use the name brand types that more heavily process the milk proteins or which offer alternatives to cows milk.
I've also used the ready-to-go, pre-made formula (I used Enfamile). However, that is way more expensive than powder formula... And I assure you, the attempt to switch from pre-made to powder can be a rough path!