I have seen theres alot of new players joining or returning players i list below twhat going prices are for upgrades . usely if you see an buck on an upgrade it means the player selling and the player who bidded has work out some kind of deal. for it ex = bidder traded an certain amount sets of  horse/dog tixs , or an grafic going prices are as followed (if i am off on prices let me know so i can corret them. 

If your wanting to sell some 6th months and one months usely are the ones that sell better

1 month =  60-70mil

6 month = 260-280mil

12 month = 560 mil