Generally - the term "28 day program" refers to inpatient.
The 28 day program is also known as the Minnesota Model and was developed by the folks at Hazelden. Just a little history for you...
I've never attended outpatient treatment. Only had inpatient - 3 times. As far as cost, figure $1,000 per day for inpatient treatment (average). Probably the most expensive "vacation" you'll ever have.