how can I increase transmit power of my yaesu ft710 and what will it cost
The classical answer to this question is to get a linear amplifier. Gigaparts can fix you up with suitable unit. If you need more from your 100 watt radio, consider a legal limit model. This will boost your signal 2 or 3 S units higher at anyone receiving you. Check the catalog for prices.
If your goal is to achieve more contacts and/or larger distance, you might consider looking at better a better antenna situation. An antenna having gain and/or a more favorable launch angle vs what ever you are currently using will help you on both transmit AND receive.
Joesph, As Robert has advised an amplifier or Bigger, Higher antennas is the alternative. Sadly Yaesu has not issued a new amplifier but when it does it will be expensive. For example the Icom ic-PW2 is here: Icom IC-PW2 1kW Linear Amplifier GigaParts.com. What is your antenna situation?
Thanks Doug W3CF