DM Internship programmes are very, very common in the dive industry. Because this is a professional level course, it makes sense to be immersed in the job, as you will learn far more from being involved in the day-to-day operations and functions of the divemaster...and establish more robust mentor relationships with the instructor/s.
I did my DM internship in the UK, only at weekends, over the duration of a year. During that time, I completed a huge amount of diving, had the chance to understand the thought process and attitudes of my instructors and gained knowledge/skills in extra things, such as gas blending, dive shop management, dive operations management etc etc etc
On a purely financial basis, the DM internship that I participated in allowed me to dive every single weekend for over a year...at no extra cost outside the initial course fees.
If you have no intention of working withing the dive industry, then you could always opt for a course, rather than an internship....(and I am sure that is offered also)... but you would be missing out on the wider breadth of knowledge available from spending time closely associated to a dive shop and being a member of the team, rather than just a 'customer'.