Actually, it's almost identical to GW2's WvW. Just like GW2, the "open world" PvP zone is simply a different zone in the game world. Just like GW2, if you are a lowbie, you will be scaled up, but "real" higher level characters are still going to be much more powerful than you due to gear and skill point advantages. Just like GW2, you go out there in your same PvE gear, with the same PvE characters.
The biggest difference is the size and scope. Cyrodiil is more-or-less 36x bigger than EB and it's all one single map, not 3 different BL maps and one EB map. Waypoints operate slightly differently too - your realm/team needs to hold all of the castles and towers between where you are and where you want to go. This opens up a neat little strategic element where a smallish group can "cut off" your transits.
There are "relics" called Elder Scrolls that give your team a small bonus to offense and defense depending on how may you control (similar to the GW2 "orbs" that were removed shortly after launch). Stealing these from the other team is a monumental undertaking and once stolen, increasingly difficult to defend against them taking them back. Unlike GW2, the game is stacked to make it easier for underdogs to "come back" than for winning teams to snowball a PPT counter and completely destroy opponents.
There are five "campaigns" that all have slightly different rulesets. Some are short and reset quickly . Others are a longer games that are more strategic. Until you hit level 50 (called VR1 in ESO) you should probably stick to the non-veteran campaign, which is limited to under-L50 characters.
As for a cheap copy, you're pretty much out of luck. Prices on ESO started climbing when the B2P announcement was made and it's back up to launch prices now. It will probably drop back down into the $20-30 range in a few months, if you want to wait.
In my opinion (and I'm no ESO fanboy - check my comment history if you want) it's worth buying at full price. The open world PvP of ESO makes GW2's WvW look like a kiddie pool. Yes, there's a lot of mindless zerging (just like GW2) but if you're a WvW player, ESO's version has a ton more "stuff" to play with.
TL;DR: Yes. Buy it.