Can the way a site is coded affect how much we spend on hosting?

Can the way a site is coded affect how much we spend on hosting?

Our website is an eCommerce store trading in ethically sourced loose
diamonds. We do not get much traffic and yet our Amazon bill is huge
($300/month for 1,500 unique visits). Is this normal?
I do know we are daily doing some database pulling twice from another
source and that the files are large. Does it make sense to just use
regular hosting for this process and then the Amazon one just for our
site?
Most of the cost is for Amazon Elastic Compute Cloud. About 20% is for RDS
service.
I am wondering if:
(a) our developers have done something which leads to this kind of usage
OR (b) Amazon is just really expensive
IS THERE A PAID FOR SERVICE WHICH WE CAN USE TO ENSURE OUR SITE IS
OPTIMISED FOR ITS HOSTING - in terms of cost, usage and speed?