比 vs. 跟 for comparisons
跟 in "你的衣服跟我的衣服都便宜"。 and "你的房子跟我的房子都贵"。is not 'compare to' but 'also/and' 

In face, 跟 doesn't contain meaning of 'compare to' . 

You thought so because in [X 跟 Y 比] ("X compare to Y" ) structure '跟' is always paired with '比'


  你的衬衫比我的衬衫好看。- 比 = compare to 
  
  你的手机比我的手机好用。- 比 = compare to
  
  [(item A) 比 (item B) + (adjective)] = [ (item A) compare to (item B) is (adjective)]


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  你的衣服跟我的衣服都便宜。- 跟 = and 
  
  你的房子跟我的房子都贵。- 跟 = and 
  
  [(item A) 跟 (item B) + 都 + (adjective)] = [ (item A) and (item B) are both (adjective)]


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  also notice on this site, examples where the adjective is at the end. There were none like that in the handouts. All were in these two formats.


But the adjective is at the end in this two formats too. 好看, 好用, 便宜 and 贵 are all adjective

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Add 跟 in a comparisons structure instead of using 比 alone:


  (你的衬衫)跟(我的衬衫)比, (你的衬衫好看得多) 
  
  take (item A) and (item B), and compare, + (opinion/ statement)


Another way to use 比 in a comparisons structure is  "compare to A, Y is (opinion/ statement)":


  [(比起)(我的衬衫), (你的衬衫)(好看得多)]
  
  [(compare to) (A), (B) (opinion/ statement)]