The cage on this old derailleur is bent (skewed) and not worth fussing with. RD TY30 Tourney Shimano Rear Derailleur
I checked and it's hard to find this exact derailleur. Does anyone know which brand will substitute in bike: Trek 230 Mountaintrack ? Looking for a lower cost part.
Thanks.

6 or 7 speed you can use a Shimano TX35. Your LBS should be able to get one. Retail should be about $12.00

With very few exceptions*, any shimano rear derailleur will work.

A few models of old SHimano Dura-Ace (expensive road racing) and newer 'Dyna-Sys' (performance mountain bike) derailleurs will not work properly if you have indexed (click) shifting.

Shimano Altus, ALivio, and Deore and less expensive models like TX35 (as mentioned above) will work fine. The only compatibility issue you might have is that some derailleurs have a mounting plate ('claw') that is held under the axle nut, and some bolt directly to the frame. If you have the type with the 'claw' then you either need a derailleur with a built in 'claw' or you need to buy the $5 adapter claw. If you have the type that bolts directly to an eyelet in on the frame then you should get that type to replace it.

Thanks guys, that's extremely helpful. Didn't know I can get parts at such low prices.