It's not inexpensive (usually) but it's good.
Right now you can purchase DiorShow Extase mascara (full size) and get a travel size of DiorShow blackout and DiorShow lash conditioning serum with it. It's a great way to get 2 high end mascaras and a conditioning serum in a convenient size for the price of one high end mascara. I find that the travel sizes are more convenient, go on better, and last about the same length of time (as far as prime usability is concerned) as a full size.
You could also just condition your lashes with olive, almond or castor oil before bed. I think it's a much more worthwhile option than trying to find a 2 -in-1 mascara/conditioner that actually works. That's like conditioning nail polish, or 2-in-1 shampoo, how well do those products ever work? It's the most inexpensive option, and probably the most beneficial too!
Sephora acturally just came out with one I forgot the name but it conditions and helps to strenghten and volumize lashes overtime. Similar to CARGO'S lash activator thats $35, sephora is $17