The problem with any "per shrub" price, other than as a rule of thumb, is that shrubs vary in a lot of ways beyond size.
How much growth is on them?
How well shaped are they already?
Are you leaving tiny cuttings on the ground or hauling off huge amounts?
Is this shrub in a formal landscape setting or sitting beside a rental home?
What obstructions are around them? Is there potential for damage? Are they in tight spots or on slopes where footing is difficult and it's more tiring?
How "clean" does the customer want the mulch or lawn? Would they rather have a lower price or a pristine look?
Can you rake/blow cuttings into lawn and mulch up or bag them?
Do you have gates and fences to deal with when moving cuttings?
Does your idea of "trimmed" match that of the customer? Do you touch them up only to find the customer expected them cut way back for the price of a touch-up? I've had shrub jobs that filled a truck and trailer, and others that didn't even require much pickup at all.
Experience will help. Since my customers know and trust me and I work fast, I just charge $60/hr and nobody has ever had a problem. It could be harder to do that with new customers.
One tip would be to charge a flat fee per month on a service agreement to keep them in shape. That way you might be able to beat an hourly rate and give them a set price. Over time, even if you did more work than expected at first, once they were in good shape you'd know exactly how long they take to do at appropriate intervals.