Everyone is different with the amount of calories they burn during exercises. I googled planks, and it said anywhere from normal resting metabolic rate to 5.9 cals/minute.
I find the fitday estimates are wildly under for me - it gives aerobic exercise as 7 cals/minute I think, when my real burn is more like 13 cals/minute (I think I usually need to enter it as vigorous rowing, or something like that). However, they may be more accurate for you, or for others.
The best way to know if you're burning calories and elevating your heart rate is to buy a heart rate monitor (HRM) and wear it when you're exercising. It will measure your heart rate and see if it's elevated, and give you a calorie burn number based on your specific parameters such as age, height, gender, etc.
HRMs are not too expensive, but they do cost some money. If it's worth it to you, you might want to buy one. Otherwise, others won't really be able to estimate your burn, even though it sounds like you're working hard.
Female, 31 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 10/22/14 = 175.0 (net: -40.2)
Last mini-goal: 175 lbs. REACHED 10/22/14 --REWARD: New wallet
Current min-goal: 170 lbs.
Next large goal: 165 lbs. by 12/25/2014
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."