Which of the two approaches to get max value from all records of object is correct?

It sounds like you have already found the answer: using LIMIT 1 will avoid hitting LimitException. Take a look at this post, however. You may notice better performance if you change your query to:

List<Resource__c> mpList = [
    SELECT Price__c FROM Resource__c
    ORDER BY Price__c DESC NULLS LAST
    LIMIT 1
]

Take a look at Working with Very Large SOQL Queries. Querying for field != null will hurt your performance.

• Typically, a custom index isn’t used in these cases.

  • The queried values exceed the system-defined threshold.
  • The filter operator is a negative operator such as NOT EQUAL TO (or !=), NOT CONTAINS, and NOT STARTS WITH.
  • The CONTAINS operator is used in the filter, and the number of rows to be scanned exceeds 333,333. The CONTAINS operator requires a full scan of the index. This threshold is subject to change.
  • You’re comparing with an empty value (Name != '').