If Ro/Di is working?

all the filters are new except the membrane. would replacing that make a difference?
because i heard membrane can cost up to $100 and if that's the case i rather buy a new with with the di?

 

Well, a Spectrapure membrane is currently on sale for $40. I would probably change the prefilters too, TBH. You would want to buy a DI addon unit, regardless, as that'll polish off the rest of the TDS. What exact prefilters did you buy?

 

That's a good price for a membrane... don't forget the flow restrictor that needs to be replaced to match the GPD of the new membrane: RO MEMBRANES AND FLOW RESTRICTORS

 

You need a pressure gauge installed just before the membrane, so you know what your pressure is. With your very poor 70% rejection rate, you either have an old membrane that needs to be replaced, or you have LOW pressure. You need 40 psi just to get the membrane to function somewhat correctly, and an average of at least 50 psi to get a rejection rate of around 90%. Until you get a steady 60 psi, don't expect to get the 95-98% rejection rate that the membrane is rated for.

 

If you do find out that you have low pressure, Air Water and Ice is having a special right now for $25 free if you spend so much...their Booster Pump Kit will work for that $25 off...and it is pre-assembled, plug and play.

 

Not sure I would spend the money upgrading a Coralife unit, they are pretty low end to begin with but its up to you.
You definitely need a new membrane, that one is way past its useful life.
Prefilters and carbons have no effect on the TDS but I would change them anyway as they need it every 6 months regardless of usage. Add it up and you are approaching the cost of a new unit so the replacements may not be the best value. What you have today is RO only so you would also have to add a DI filter too.
Membrane- $30-$40
Flow Restrictor- $5
Prefilter- $6-$12
Carbon Block- $10-$13
Add on DI- $40-$80 depending on what you get

A new RO/DI with a tested RO 98% rejection rate membrane, 0.5 micron prefilter, 0.5 micron carbon block, full size vertical refillable 20 oz DI, inline pressure gauge, dual inline TDS meter, capillary tube flow restrictor and more is on $145 complete and warrantied.
SpectraPure Customer Appreciation SALE! 20% - 50% off

Pressure does play a role in RO membrane performance as does water temperature but if you have 40 psi or more at the tap you should not see 70% rejection if the membrane is any good.