First, join the Card Games & Designers Guild group on Facebook. They discuss fine-tuning play mechanics all the time.
In general, that group doesn't think CCGs are that profitable or popular anymore, except for maybe the most ubiquitous IPs. They say the trend is to make a game that people can pick up and play without searching for rares.
Anyway, for playtesting, I'd recommend printing demo cards, but in an affordable way. Use illustrating software to make a template outline of a card, Then type stats and text on the templates and print them out on blank paper.
Then buy a bunch of cheap common CCG cards at a hobby store.. Also, buy clear reusable plastic sleeves for those cards for around $10.
Slip a dummy card and your printed-out template into the sleeve, and voila: perfect prototype cards!
Once you're more confident about your playing rules, you can add some art to your templates and print them out cheaply at a print shop (or even a FedEx).