To make it simple, it's all about suply and demand, but what makes it different than the old Splinterlands time ?

First and foremost would be because the current modern format have less cards on the circulation. Chaos legion is just a massive because it launch on the bull season when we are around the ATH. Many schoolarships are opened and they open a lot of CL packs to provide the cards. For the current rebellion and conclave arcana, we are no longer have those schoolarship things anymore, so yeah, the suply is very little.

The second reason that makes the rental market become more expensive is that we are have to rent for the whole season now instead of 2 day minimum like was implemented before. With the old 2 day system, some people are only rent while there is a tournament and brawl, but now, they have to rent till the end of season !

EoS is the Time !

What is EoS and why EoS ? EoS stand for End of Season, which refer to when a season is end, and so any rental because it's basically become all rental period. At EoS time, we have a chance to get cheaper cards because some people not renew their rental contract, so it will be back on the market again ! My tips is to aim for the most important cards that you want to use, for me it's the summoners, then you can go to monsters after that.

Preparation is Key !

Preparation is always important on a war, including the rental war. As wise man said failed to prepare is preparing for a fail !

Before the End of Season, I personally have some preparation :

1. Put alarms, so you won't forget it (because you have to wait 2 more weeks for another EoS !
2. Get enough DEC for the rental, try get as much as you can so you can afford all cards that you need. It would be a shame when finding a cheap cards but failed to rent due to lack of DEC. Remember, this is a war, it might not available on the next minutes ! The rental period which is 15 days mean you have to pay the rental cost x 15 for each cards, so try getting lot of DEC !
3. Open the rental page with the filter before the EoS, so you can keep refreshing it and try become the fastest !

Continuous Act is Needed for Best Result

For better result, I suggest you to renew your cards before EoS, so you can focus hunting for the cards you don't get at previous season. This way your deck will slowly complete with the cheaper cards ! The other reason is because sometimes, things are happened so you can doing rent at the EoS time. FYI, this might cause huge gap on the price different rather than rentil on the next day (not EoS) !

Here is the example of Nidhogr that I rent for 30 dec/day , and how the available is on 90 dec/day. It's triple price ! So, you can save a lot by renting the cards on the EoS period !

Another Example of how the rental price is triple than the one I got on the EoS time !

Final Words