I also had this issue when first starting my exploration of AR/VR in the classroom. Phones that fit the specifications for usage with CoSpacesEDU and VR/AR apps in general were way too expensive for the numbers that I needed. I was pulling my hair out and every search that I conducted always brought back phones, phones, and more phones! I finally learned from a DonorsChoose.org colleague that iPod touches (6th generation) worked perfectly fine and the price point, although still seemingly high for classroom teachers, was much more attainable than general phones. I now have six of them and they work awesome with CoSpacesEDU! Even one device is more than enough to engage your students in an immersive experience. VR is definitely the way to go for the future and I am preparing to introduce my students to the HTC Vive system! We have been fortunate in being able to fund "dream" projects via DonorsChoose.org so if you are a teacher in the U.S., definitely check them out! You can check out my site here:
www.DonorsChoose.org/YodaPond and look at my past projects. Hope that helps!