It is better you invest low amount of money on bitcoin right now. If your salary is $400 monthly, you can be investing like $20 on bitcoin which is 5% of your monthly income .
I think you misinterpreted something from Op. Op meant he/she has will be able to invest $300-$400 monthly, i don't think this means he/she earns that figure monthly.
The price is high already but there is tendency that the price might go up. If you can be patient, wait until the next bull market begins after the coming bear market to invest more. I may not be absolutely right but that is my opinion.
I think this is dependent on the Op investment plan, how long does he/she wants to HODL the coin before taking profit.
While the price of bitcoin seems like crazily high now, people current view of the market shows even after hitting $100k many will still be HODLing maybe probably to the 150k. This means Op can still invest now but that doesn't means he/she should just go all out because can go side ways at times and so it's important to invest, what will not make life difficult for you in order not to sell your coins in lost. Also, if your plans is actually a long term like 5 to 10 years, you won't have to worry much about the current price because there's an high tendency of it already passing $200k by then.
Exactly 300-400$ is not my salary but a budget I can invest and afford to lose .
I want to make a long term investment here .