The easiest way to get this achievement is to go to the Scrollway level and farm the bonus round there. Boost over to the bonus round, go into it, get 4x coin multiplier, grab the big gem, restart the level. Rinse, repeat. You can get about 250k coins in 15 minutes or so of farming. However, as only 8 boost colors are available in the shop at any given time, and the store takes about 15 minutes to cycle, you're going to want to spend some money on other things as well along the way. Be sure to take a break after every store reset from farming to buy more items.
You'll want to buy the necessary stuff to complete the Big Money, Shoe Shopping, Head Hunter, and Rainbow Rex achievements along the way.