The Starter Pack retails for $99.99 which includes plenty to play with. It is higher than other Portals, but again you get to play with those legos anytime.
There are many expansion sets too, Wave 1 is on shelves with more waves to come. Jurassic World would be my favorite to start with. There are plenty other collections with DC Universe, Ghostbusters, Back to the Future, The Simpsons, just to name a few.
LEGO Dimensions is a big investment, but its highly worth it!