First things first. You need to know how or learn how to skate. Good inline hockey skates generally run between $200 and $300. You can also upgrade the bearings in the wheels. I would go with abec 5s or above. You can also go with swiss bearings too. Abecs run around $25-$35. Swiss run around $45-$55. You will also need shin pads. Expect to pay around 25-$30 shin pads.
Next thing you would need are gloves. I would pay around $30 for gloves, but you can go higher. The stick is also important, wooden sticks can be around $20. You can also buy stick shafts with replacement blades, so that if the blade breaks, you can replace it. A cheap stick shaft will be about $40.
As we wind down to whats needed, you'll notice I didn't mention elbow pads. I don't use em, they aren't really essential, although some leagues do require them. Helmets are the last item you need to get running 60-$80. It is up to you weather or not you want to get a cage for the helmet. Oh and I almost forgot, you will need inline hockey pants which can run 40-$70 depending on the brand that you get.
Now if this all sounds too expensive, you can go cheap and not get the most pricey equipment. The way i started was, I had a stick, then I got skates, and then I got shin pads. Everything else came after awhile of playing. This was just with friends of course, if you want to join a league you will need most of the items above. Indoor leagues are especially strict about the type of wheels you buy, so that can factor in on cost as well. Like I've been saying all this time, hockey plus equipment equals expensive.