In simple terms
H81 entry level mobo
B85 basic mobo
H87 intermediate mobo
Z87 Semi pro mobo to Pro mobo
Anyway each chipset offers different functions. Most people will fall into into either a h81 or b85 mobo. While some will ended up in h87 and z87.
Apart from itx H81 offers only 2 slot rams, 2 sata 3 ports and 2 sata 2 ports, 2 usb 3.0 front and 4 usb 2.0 front and back. Certain things are taken out depending on various brands from the back io. Like some have vga, some have dvi while others dont, it varies from brands.
B85 is a slightly more features over h81. It has 4 ram slots, additional 2 more sata 3 ports. Has slightly more connectivity thru the back io.
H87 offers 2 more Sata 3 ports. It also added in a few more features like SSD trim functions, RAID functions for connecting multiple storage drives for faster performance in raid0. Or other raid functions for redundancy etc.
Z87 varies. But common stuffs are overclocking of cpu is allowed in pairing with a "k" processor. Certain lower end z87 comes with very little power phases such as the z87 g41 pc mate. While higher end ones provide more power phases. On lower end z87 they come with only crossfire support for multi gpu. Just like b85 and h87 it supports only x8 and x4 bandwidth. Only high end ones will give both at x8 x8 bandwidth with added functions of SLI from nvidia. Different board manufacturers also have certain unique features on their boards. Like audio boost from creative. Asus have some io device to control their or monitor their board status. Etc.
Normally what we recommend here is if you are on a tight budget, h81
Normal usage pattern with slightly more budget b85
Those whom needs RAID features h87
Those whom wanna play with overclocking and multi gpus config a z87
So to your question $300 for mobo and cpu is just gonna be a h81 chipset board with a i5 4430 4440 or 4570.
Asrock h81m dgs + i5 4430 ~ $287
$300 for gpu arranging from lowest to highest performance
Gtx750ti
R9 270/r9 270x
Else nothing much.
