Yea bitcoin price may be too high, but doesn't mean we can not buy some fraction. It is not a must to buy a whole bitcoin as bitcoin could be bought in SATs. I believe you are not a regular poster of this thread otherwise you will know what is called the DCA, lump sum and dip and HODL approach or strategy. Investing as low as $100 in bitcoin should be done in a right approach or when you have know the strategy to invest without overdoing it.

1. Depends on your level of income generation like the amount of salary you receive. If your salary is within the range of $100, you may not be able to Invest that $100 at ago. You must surely figure out a way to divide it in 3 or 4 different part. investing like %20 in bitcoin, setting aside %15 for emergency.  %25  for discretion and %40 for feeding and other purposes. Note this my analysis may differ from your own not necessarily for everyone but just like example because your Level of responsibility may be higher to the extent of using %60 of you salary for feeding to be able to invest in bitcoin freely without touching your btc Investment.
2. You must be consistent and having a long time goal making sure you buy bitcoin every week through DCA for a longer time Horizon which you have programmed. Maybe 4--10years and 20-30years.
3. Always make sure you have a steady income in your reserved to be able to keep accumulating every week in Oder not to sell your HODLings because our fear is not to sell off our bitcoin HODLing while we haven't met our Target.
4. investing in Bitcoin shouldn't be done in a situation that will affect you otherwise know as aggressive investment. A situation where you invest an amount that will affect your investment approach. For example having $100 and investing $80 and managing $20 is surely an agresive investment. Expecially when you don have another source of income from anywhere.