How much space are you talking about? And yes, you sort of answered your own question by saying you know the cost will depend on the particular landscaper, and what plant you'll use.
To give you some perspective, My mom had a few plans and estimates done for a house she had. The areas she wanted done were fairly big. She had estimates ranging from about $800 to $8000.
Thinks to think about when you call around, are:
1) Do you want easy, low maintenance? Are you willing to put in a littel time now and then for some really wonderful things? Or would you settle for some less interesting, "durable" plants & shrubs (like the ones you already have) that you can ignore.
2) Do you want flowers? shrubs? Flowers you can cut and bring inside to enjoy?
3) How about trees? - Deciduous? Evergreen? Shade?
Possibly go to a nursery, and take a stroll, with a little note pad. Write down the names of plants that catch your eye. You can bring these up with the landscaper. You may also get ideas about the costs of different things.