Broad targeting - Can you guesstimate future results?

[Ads newbie here]

Basically I am testing a niche B&M product, and finding some traction with interested leads. I want to pull the trigger on this but I am worried about running out of interested people after a couple of months. Is there a way to estimate size of niche audience to predict long-term CPL?

Here are some testing results from campaigns I ran:

• Broad Targeting Original Audience - Size: 300K
• Reach: 12K accounts
• Frequency: 2.0
• Leads: 8
• CPL: $35
• CPM: $13
• Link CTR: 0.6% (I definitely have to work on this)
• Scaling target: 15-20 Leads/month

I understand that scaling will raise CPL over time. I am okay with it increasing gradually to $60 over a year time, but will I suddenly run out of clients and hit > $100 CPL after few months? Is there a way to guesstimate future trends?