Cost difference between downloading S3 files into Lambda function via HTTPS VS SDK?

if you access via https://s3- you need to attach a signature to download the object or having the object public. In the case of s3.getObject the user/role that is downloading the object just needs the permission s3:getObject of the object you are downloading.

s3.getObject of the sdk is a wrapper of the REST Api call for S3, so overall they are both making an https request, but all depends from your case.

If the object is public and you don't want to use the sdk, then go with https://s3- but if your object is not public I would suggest to use the sdk as it makes easier and cleaner (from code point of you) to download the file.