JPR and Mercury research define it as under £300(if you convert from USD) since another name for midrange is mainstream. Enthusiast level starts at £300 onwards IIRC and AMD and Nvidia use these companies for marketing bumpf. Midrange is price defined,otherwise it would be like saying since Ford has a GT40 which costs 100s of £1000s,a Ford which cost £50000 is budget territory(The average price of a car in the UK is under £30000).
It isn't though as luxury tier products are not indicate of what is considered midrange,since midrange,or better known as mainstream,is done mostly on price. For instance dozens of market research companies,phone companies like One Plus,etc consider high end phones start at £350 to £400,and everything under is midrange/mainstream or low end/budget.
Except,the market above £400 goes upto £10000 for phones. Its only enthusiasts on hardware forums who have this weird notion that just because a top card might cost £1500,suddenly all the stuff below it has to move up tiers. It does not happen with most tech products or most stuff in the realworld. High end and luxury tier products are targetted towards people where pricing can be more elastic. Mainstream and budget is more price limited due to the people targetted.
Hence why on Steam most cards are under £300. There are more GTX1060 users than all the GTX1080TI,GTX1080,GTX1070TI and GTX1070 users combined,and that is a survey which is more likely to be answered by people who have some interest in hardware.
Edit!!
https://www.anandtech.com/show/12499/q4-2017-discrete-graphics-card-shipment-report
In that report from last year,JPR defined high end as $250+ and only 16% of sales last year during the mining boom were over $250.
Mid-range is defined as $150 to $250 and over half of GPU sales happened in early 2018 in that price range.
Even if you said $300,its still under £300.
Midrange and mainstream do not mean the same thing.
None of us here will deny that the Turing range of cards have bad prices, but the 2060 is the middle of the range and in my opinion it is just a tad too steep to really be considered a mainstream part.