Dynatrace charges $0.001 per synthetic request. If you configure an HTTP monitor with 1 min interval from a single location then the monthly price will be $0.001 * 1440 minutes per day * 30 days in a month = $43/month.
Sematext, on the other hand, is only $2 per HTTP monitor, good for up to 200,000 runs, and $7 per browser monitor, good for up to 15,000 runs. But, if you have more than 16 monitors and opt for the Standard or Pro packages, the price per monitor will go down. The Standard plan starts at $29 per month for 40 HTTP Monitors and 5 Browser Monitors, while the Pro plan starts at $99 per month for 100 HTTP Monitors and 15 Browser Monitors.
Overall, with a single HTTP monitor, you're looking at a minimum of 95% savings with Sematext.