I've used NGK plugs for over 30 years and will continue to do so with pleasure. Depending on which model plug they sold you, will have an effect on pricing, but in my opinion $15 per plug for the Iridium plugs for the X, are still too expensive especially if they didn't come in the OEM supply Subaru Blue & White boxes, complete with part number.
Have a read of the NGK website for all they offer. NGK, is a OEM supplier to Subaru. When you get OEM replacement wire sets they are the blue NGK models, also available cheaper through automotive supply chains.
Coming from someone who likes my dealership and is a friend of one of the mechanics, sometimes the aftermarket offering is identical to the OEM part supplier only cheaper, and if I'm DIYing I'll go with the less expensive ones.
Just to get some reference of US prices Auto Zone does list the Iridium plug for $6.99 on their website, probably special order at some stores. I too use the more standard G-Power plugs with no issues and can get 4 of them locally for about $17 CDN.
If you are like me, I like to check them after the winter to see how they look and change them as part of a simple seasonal tune-up, probably overkill based on how long they actually will last, just a habit I'm not concerned breaking.