You are on the right track. Starting at 2x and increasing your price is awesome!
Most of them will want to "assess the value" first or something else to keep the conversation away from money, at first. They want to try and hook you, then low-ball the fuck out of you. You need to confuse their routine at all costs.
Flipping the script will usually confuse them. If you are familiar with high pressure sales, use everything in the book. Sob stories, FOMO, extreme sense of urgency, etc. Start pressuring the fuck out of them to buy and don't let them distract you with stupid shit.
Now that I am thinking about it, I haven't gotten one of those calls in months. I started dumping pages of XSS and SQL injection test scripts back at automated texts so there is a chance I broke something. Dunno.
You can find payload lists out there in the wild. Here is a repository filled with them: https://github.com/payloadbox
We use lists like these, which are generally benign, to test websites for vulnerabilities. My theory is that the software they use to manage these text messages is probably web based and not designed for this kind of input. XSS like this, if executed, could cause an endless stream of popups on their side similar the days of the wild wild web. It's not going to hurt anything, but they won't want to reference my text logs any more.
Obviously, there are a ton of caveats. Depending on how the message is secured in transit, your carrier might block it. I dunno as I have never worked in the mobile security space. You might piss your own phone off. You might break your own message histories..
There are a ton of unknowns, btw. I personally don't give a fuck about any of them.