Another thing merchants don’t consider when going the cheaper route is the value of their own time. Time is money so consider how much time you are going to have to spend on your project if you don’t make the right choices for your site.
The best example is Ikea. The reason they are able to charge so little for their furniture is they effectively pass on much of the work that other furniture manufacturers would do, to their customer.
Once you have chosen your new bed, you have to pick it, pack it, deliver it and assemble it yourself. Clever. In addition, most of us are not professional flatpack assemblers therefore the quality and the output is not going to be as good as it looks in the catalogue or showroom. How many times have you put the wrong screw in the wrong hole or found out that you don’t have the right tool to do the job properly?
It is precisely for this reason that a whole industry of services has grown up around Ikea that can provide you with anything from delivery/collection to assembly of their products. Ironically, by the time you have paid for the product and the services, Ikea furniture is not that good a deal anymore.
That metaphor applies to website design and build so make sure that you hire a professional agency that knows about user journeys, personas, conversion ratios, integration, security, performance, online marketing, etc. If the agency you choose does not have these skills, you are going to have to fill these gaps yourself or pay someone else to do so.