Marth's map costs 300 yen in Japan and $2.50 in America. We can estimate a conversion by just swapping yen to cents and then taking off $0.50.
With that in mind, let's say you get Marth for free, then nab Alm, Eirika, Ike, and Katarina, the ones I'd say are must-haves. That's $13 - quite reasonable. Anything else is a nice bonus if you so desire it.
And of course you don't even need any of those to have an experience more complete than any past FE game, but who could pass those up?