I am not an ETP,but have a $6.95 account, however i think I only paid about $100 for it at the time. I have accounts elsewhere with prices from $6.50, $6.75, to $8.99, and weight the cost of higher renewal with features and the ease of not transferring all over. Somtimes it's just easier to pay the $8.99 renewal than go through the hassle of gettign auth codes and doing all the data entry.
There is the risk of Enom ending the grandfathering of non-ETP accounts, so I personally wouldn't invest more than I would benefit in a year or so. There is also some risk of the reseller above you raising your prices,and some tips around to help stop that from happening. The only way to control your own destiny is to be at the top of the food chain and deal directly with Enom. However, there are other alternatives out there without the high initial cost of the ETP. Registerfly has similar pricing for 500 domain packs, but the prices don't stay down forever and you have to keep buying them. comparing an ETPaccount, at least every dollar (except 3% credit fee) goes directly toward domain reg or renewals. With a non-etp acoount, none of what you pay goes toward domain purchases.