I wonder how you were able to get a construction bid without a plan?

At any rate, without seeing your site it's difficult to know why it would be so expensive. If there are access issues so that a lot of hand labor is required, the costs will increase. Do you have any irrigation already installed? Do you need drainage? Lighting? Soil amending?

For design plans...it will depend. Landscape architects or architects are the only ones who can legally provide you with construction details, unless you go with a contractor who also installs what they have designed. Do you need permit drawings? A great set of construction drawings are worth their weight in gold if you can get a number of bids to choose from...and are very detailed so that there are no questions as to what needs to be installed. Better to pay one designer than paying six workers to stand around not knowing what to do next.

It is really helpful for your designer to know what your construction budget is so they can design what you can afford and/or phase in. A rule of thumb is that the landscape will typically be about 10-20% of your property's value.

Hope this helps!