I would say looking at sites such as theme forest etc that the average cost for themes is around $30-40.... It all really depends on specifics such as business application, features etc so you could be paying much more...
I recommend that you check out themes whereby they have drag and drop content builders plus templates as that may meet most of your needs and thus minimizing efforts with additional contractor help for implementing further custom designs and enhancements... Check out themes such as Divi, Xtheme as well as thrive themes as they are extremely flexible...
Regarding design contractor help - that also depends on the amount of effort vs. requirements etc... I would recommend that you use an escrow service for payments to ensure fair play as well as partial payment on milestones during development so that you can keep track of what is being done through reviews as well as amending requirements which may change....
I would say for small-medium layout changes you could get a designer for around $50-100 on sites such as odesk, freelancer etc... If you can perhaps estimate the number of hours to do the work and apply around $20 per hour, noting hourly rates differ massively between countries.. If I was you I would place your statement of work and ask for quotes...
Hope that helps! Best of luck, Sid