Costs depends on the features and also the developer, each developer has different rates and skills For a basic builder i think 1000 - 1500 usd would be fair. With this i mean the very basic, Drag and drop, edit texts, export option.
Ps you want to develop a themebuilder to edit newsletters or other types of files?
Thanks for your quick reply and your opinion. I only need this template builder for newsletter… Feature should be like yours…Drag& drop, Edit text, color change, export to Mailchimp and Campaign monitor option…