Since money is the issue, the Lava Tube will be an upgrade over your 900 mAh Egos. You'll likely not think about the Provari you could have saved for unless the Lava Tube stops working or breaks. If it happens to be out of warranty, you'll be shopping for another mod real fast as they can not be repaired.
The Provari is not only a great durable and dependable mod, but in the unlikelihood should it need repair, it comes with a 1 year warranty. Even after the warranty expires, the manufacturer will service it for very reasonable fees for as long as you own it. The entire process typically takes less than a week.
After Warranty Standard Service Repair Costs:
Replace Window and screws- $10.00
Replace Circuit Board - $65.00
Upgrade to V2 - $20.00
Replace plastic pushbutton - $5.00
Repairs to the circuit board - $25.00
Cleaning - $20.00
Replace top cap - $15.00
Broken Cartomizer Service - $10.00
Return shipping within the USA - $5.00
I look at owning a Provari similar to an investment in a high quality power tool. Sure, you can get by with Harbor Freight quality (Lava Tube) or you can pay more for DeWalt or Milwaukee quality (Provari) which will take abuse and last years under hard usage.
You are paying more for a Provari because of higher quality parts and construction, USA made parts and assembly, attention to detail, a longer warranty, lifetime customer service, and the ability to have it repaired quickly for inexpensive cost. Is that additional money worth it to you? Only you can answer that.