Good studio monitors exist so you can hear audio clearly, not to sound artificially bass-heavy like consumer desktop speakers. Buy a solid pair of active monitors once and they will easily last you a decade. That means skipping the ultra-cheap entry gear you will just want to replace in a year.
Pro audio brands run on predictable release cycles. When a manufacturer drops a new generation, last year's model usually goes on sale despite sounding almost identical. Timing your purchase around major retailer sales in November lets you stretch a $650 budget much further.
Keep in mind that an intro window is a fixed deadline, not free money. If you carry a balance past the 18-month mark on your Citi Simplicity Credit Card, standard interest rates kick in on whatever amount remains.